Gilbert Ellis kicks himself every time he thinks about how he lost Australia.

That happened back in 1972, just after he stepped into the presidency at Household Finance Corp. Ellis had decided it was time for HFC to expand overseas. In fact, he feared it was already past time. But, unable to expand everywhere at once, he faced this choice: either Australia or invade England. Figuring Australia would be around for awhile, and drawn by England's larger market, Ellis bet on England.

Soon afterwards, Australia's atitude toward foreign investment soured, effectively spoiling HFC's expansion plans there. "If only HFC had gone overseas sooner . . . We missed that opportunity," Ellis said during a recent interview. "It still gets to me."

Household this year is celebrating its 100th anniversary. Ellis, 62, is now chairman and sounds determined not to let opportunities like Australia slip away again. But first, he has some catching up to do.

In the past 15 years, HFC has lost more than Australia. Its consumer finance subsidiary has not kept pace with vigorous growth here at home of the consumer loan market. Alhought HFC remains the largest consumer finance company in the U.S. with 10 percent of the finance market (it loaned a total of $2.9 billion last year to 2.5 million borrowers), the firm has been slowed in its tracks by the aggressive expansion efforts of banks and credit unions.

Part of the reason for HFC's lackluster performance in the domestic market - and its failure to get a jump on expansion overseas - can be atributed to heightened competition. But part, too, is due to the strain on the company of an ambitious diversification program.

Beginning in 1961, Household moved deftly and methodically into a variety of new and, for the most part, profitable business. Today, HFC makes thermoses, sells groceries and rents cars in addition to lending money. Only 46 percent of HFC's net income last year of $139 million came from consumer lending. A favorite tag line around Household headquarters here is that 8 out of 10 Americans buy from HFC, though most probably don't realize it.

No expansion, of course, ever comes without cost, and HFC's bedrock consumer finance division found itself having both to help finance the new business and curb its own growth. This shift away from HFC's traditional business into retailing and nonfiance areas was orchestrated by Harold E. MacDonald, a former executive from Montgomery Ward & Co. who served as Household's chief executive offier though most of the 1960s.

With the rise Ellis, who spent 42 years climbing up through the ranks of the consumer finance division, Houshold has given notice that it will be returning with greater force to defend and expand its oldest, namesake business.

Ellis insists he's not playing favorites in his company. He and HFC's young president, Donald Clark, 47, whose background also is primarily in finance, stress that HFC is no longer just a finance company and that all divisions have been budgeted for growth.

Clearly, though, the push is on in finance - with several new twists. In addition to lending money, HFC's finance subsidiary has started offering a variety of financial services, including writing insurance, preparing tax returns and even issuing savings certificates. Some of these services have been introduced through HFC's existing 2,000 branch offices, with the idea eventually of turning these centers into financial supermarkets that would handle all the financial needs of a family.

But this concept is still largely in its drawing-board stage - and still ahead of the legislation which would permit loan companies such as HFC to branch into other financial services. Household's neighborhood offices continue to specialize in consumer loans, mostly for $3,000 or less, carrying interest rates several points above the 18 percent generally charged by credit card companies.

At the same time, HFC has bought several financial institutions, planning some day - if Washington allows - to merge them into a completely integrated consumer financial system. In the last six years, HFC has purchased two insurance companies (Maryland Life and Insurance Co. and Alexander Hamilton life Insurance Co.), a California savings and loan association and a string of industrial banks in Colorado. It has launched its own income tax service and recently set up a commercial leasing operation by buying the staff of American Fletcher Leasing Corp. The company also expects to expand its office network by acquiring the American Investment Co. for which it has a tender offer outsanding worth $76 million.

This flurry of activity has been described by one competitor as a "shotgun" approach. But HFC officials defend their strategy claiming they have kept their business in perspective.

"You can do a lot of things if you're doing it all in moderation," said Clark. "We did not go out and acquire the largest S&L, insurance and tax preparation companies. There is a tie to each and they are manageable. If circumstances change, we could still reverse our course."

Overall, Household's revitalized finance strategy has shown mixed results. Profits for the finance subsidiary jumped 9 percent in 1976 and 16 percent in 1977, but have edged up only one percent for the first nine months of this year. Company officials blame 1978's poor performance on the squeeze of higher interest rates.

The market itself has shown a tremendous spurt in recent months, as consumers have gone more heavily into debt to finance a buying spree sparked by the worry that inflation tomorrow may be as frustrating as inflation today. One important measure of consumer indebtedness is the ratio of consumer debt to disposable income, which topped 20 percent this year, causing alarms to sound in some financial corners.

AT HFC, the average loan size has ballooned from $1,258 in 1973 to $1,544 last year. But company officials say they aren't worried about a sudden drop-off in business or a massive round of defaults.

"We do not believe our customers are overburdened," declared G. Jack Downing, an HFC finance vice president. We have a pretty good idea of our customers' ability to carty a loan." And in fact, the company's loan delinquency rates have been falling.

Chances are, if HFC sensed something going awry in its loan business or in any other of its businesses, it would be the first to apply the brakes. For in tone and character, Household is as it has long been a midwestern-brand conservative, by-the-book basic company.

Asked how he would describe the flavour of the company, Clark answered, "vanill," which though said jokingly, was very apt. There is a certain wholesomeness, a lack of the startling or particularily exciting about everything HFC has gotten into. Most of the products it makes are everyday items - thermoses instead of radar ranges. Most of its retailing and wholesaling stores are located not in urban centers but in small towns.

HFC's other business today include:

Merchandising. HFC owns two retailing chains - T.G.& Y. Stores and Von's Supermarkets - that each will achieve sales of more than $1 billion this year. It also owns two successful wholesaling chains. Ben Franklin variety stores and Coast-to-Coast hardware stores. It has one major trouble spot still: White Stores, an auto parts chain which last year showed a loss of $46 million. But a number of White Stores have been closed and financial controls instituted, allowing HFC officials to speak with guarded optimism about the chain.

Manufacturing. HFC this year will manufacture more than $200 million worth of such familiar items as Thermos thermoses. Structo barbecue grills and Scotsman ice makers.

A major stumbling block to further growth in this division has been HFC's lack of success in acquiring other manufacturing companies. "We haven't found the kind of company we're looking for that can be bought at a reasonable price," said Robert Trow, president of HFC's manufacturing subsidiary. But HFC may have been playing too nice in a national merger gaem that has become increasingly aggressive and mean. The company's policy has been to avoid going after companies that didn't want to be bought. That is changing.

Car rental. HFC owns National Car Rental System Inc., the third largest auto renting business in the country with 21 percent of the market as against Avis' 27 percent and Hertz's 39.5 percent.

There has been a definite method to the diversification palm HFC has pursued. "We started with the concept we were a retailer of dollars," said Household's president Clark. "It was easy to go to the concept of being a retailer of merchandise."

But now in its centennial year, HFC is bothered by a corporate identity crisis. Each of HFC's retailing chains operate under its established name, so do the varied manufacturing companies and so does the car rental company. The consumer tends to associate the familiar HFC ciruclar logo only with the company's loan offices.

HFC officials say they have considered changing the corporation's name from Household Finance to something like Household Industries. Recently, they went as far as hiring the consulting firm of Booz Allen & Hamilton to study HFC's corporate identity.The study cautioned against a name change. The matter has since been tabled.