Virginia Senate Majority Leader Hunter B. Andrews, responding to reports that Gov. Charles S. Robb had collected more than $50,000 in undisclosed contributions after his 1981 election, said today he will introduce legislation that would require political candidates to report all such donations in the future.

Robb said later through a spokesman that he would support the bill and that he was instructing his chief fund-raiser, Del. Alson H. Smith (D-Winchester), to make a complete, voluntary disclosure of the unreported campaign funds. Smith said the disclosure, which will include the amount and source of each contribution, will be released Wednesday afternoon.

"The principle of our laws is that the public is entitled to know what the contributions and expenditures are by a candidate," said Andrews, a Hampton Democrat and chairman of the Senate Privileges and Elections Committee. He discussed his proposed bill with Robb during a breakfast with the General Assembly leadership at the Executive Mansion today.

The problem, Andrews explained, is that existing law only required postelection reporting of contributions and expenditures by campaign committees with outstanding debts. Since Robb's committee had wound up the campaign with a $30,000 surplus, the senator said its failure to disclose activity after its final Dec. 15, 1981 postelection report "didn't violate the law in one bit."

"Frankly, we never envisioned campaigns having surpluses," said Andrews. "And it isn't just the Robb campaign. They're plenty of people who have surpluses."

According to The Newport News Daily Press, which first reported the existence of the undisclosed Robb contributions, 48 members of the House of Delegates have spent more than $67,000 from campaign committee surpluses without disclosing it to the Board of Elections.

Several delegates have also acknowledged receiving undisclosed contributions. Del. Richard Bagley (D-Hampton), the chairman of the House Appropriations Committee, says he took $1,000 from the Newport News Shipbuilding and Dry Dock Co. last year without reporting it because the contribution was made after the election and his committee had a surplus.

In a case similar to Bagley's, the Robb for Governor committee continued to raise and spend money throughout most of last year without filing reports with the state, Smith and committee accountant Anthony Barone have acknowledged in interviews.

The undisclosed contributions came in from mostly business sources "for some time" after the Dec. 15th cutoff, Smith said. An estimated $4,000 is reported to have come from a postelection fund-raiser in Chicago and another $10,000 was turned over by the state Democratic party.

In addition, an estimated $25,000 which the Robb campaign committee had loaned to Robb's inaugural committee was repaid with interest, while $14,000 left over from a separate "Friends of Chuck Robb" committee established before Robb declared his candidacy was also turned over to the campaign committee. The campaign committee paid about $7,000 in federal income taxes in 1982, Barone said.

On Jan. 5 of this year, the committee transferred $96,015 to the "Virginians for Good Government Inc.", a political action committee created by Smith that will be used in part to finance Robb's political travels. It will also raise additional funds and make contributions to state candidates who have a "good, solid philosophy" and believe in "good government for Virginia," Smith said.

The disclosure of the unreported political contributions and the new PAC embarrassed Robb aides today and elated Robb critics. One Democratic senator privately compared the PAC to a political slush fund.