"Go ahead," Colleen McCullough proffers a pack of cigarettes, "Take one. Take the whole lot, if you like. I can afford it."

She leans back and laughs heartily - a fat lady's laugh. "All 200 pounds, five-feet-10-inches of me," she says offering her statistic in the same way she offers her cigarettes - as if they were a gift of little consequence, but one that she enjoys giving. "I don't like having my picture taken, and I'm not used to it. To like it would be terribly narcissistic, I think. And I'm no beauty. Nobody who's been fat all their life could possibly be narcissistic. But I've always had tons of boyfriends. Apparently men find me terribly sexy. I don't know why but they do. And I've always attracted the good looking men, too. This may sound unkind, but I never had to content myself with men nobody else wants . . . So ther's hope for the fat."

She laughs again, shifting masses of iodine-red hair that stream down to her waist. "Gene Shalit," she says grinning, "said to me on the 'Today' Show that if I ever cut my hair, I could make it into a shirt for priests. I though it terribly funny, really."

A little explanation is in order before she races on. Colleen McCullough has just come out with an Australian novel called "The Thorn Birds," which is all about three generations of women in the red-headed Cleary family. And yes, she can afford a few cigarettes, her book having being auctioned off to Avon Books for $1.9 million, which is a record in the book-biz. It is really the women who count in her 330-page book, but since it has as its sex object a darkly, wonderfully, unbearably handsome man, publishing insiders like to compar it to "Gone With the Wind."

But the gorgeous hunk, in this case, happens to be a priest. Now you can just imagine the heartache this causes McCullough's heroine who yearns and aches and gets - only momentarily - what she wants, because, as the author says, "He sticks to his guns." And you can also imagine the kind of queries Australian-born McCullough is getting these days.

"Nev-ah," she says Definatly, when asked if she ever had a yen for a priest. "No, I plead not guilty to that. Maybe if I'd met the rigt priest at the right time, but -" she shakes her head - "I just never did."

"Of course I went to a convent school in Sydney for 12 years, and the curates were very handsome. But I was more of a man-hater then, and I despised men. I don't now, at all. But then I was probably under-sexed," she explains cheerfully. "No, what happened with my priest was this: I began writing this book on Friday the 13th, which is my luck day, in 1975. I'd just had a very unhappy love affair, and I as miserable when I began writing. Now whether or not I invented Father ralph as a substitute, I just couldn't part with the man. So I tied him up with the Cleary family."

The "Thorn Birds," as it happens was published last Friday - the 13th, as it also happens. Which didn't prevent some book stores from selling it before then - 200,000 copies so far, and that's still in Harper & Row hardcover. At the time of the interview the author was staying at the Fifth Avenue apartment of her Harper & Row editory, Ann Harris. And Ann Harris was gazing at her fondly, bustling about with coffee, with cigarettes, with preparations for the party in McCullough's honor that was to tka e place that night.

"I had no idea the book (would command such a price)," says McCullough. "Did you Ann? I always meant to ask you that - did you know?"

"Well I was excited enough to tell people in the house," replies harris.

"No, Ann wouldn't tell me something like that," says McCullough. "Because she wouldn't want to get my hopes up. She knew how poor I was. I was 30 before I began to earn a subsistence salary." Despite the critical acclaim that greeted McCullough's other novel, "Tim," for most of her life she has been a neurophyslological technician (she received a bachelors in science at Sydney University). From 1967 on, she practiced her trade in New Haven. By the time she left last year she was earning $12,000 a year. And she had to support her mother in Australia, these last few years. Colleen McCullough could not afford to take off a year to write a novel.And moreover, she didn't want to. "My 9-to-5 job is my contact with people," she explains. She tried not working once last year and went "stircrazy."

So what she would do is this: First she would spend eight hours at her job at Yale. Then she'd go home to tap out. "The Thorn Birds" - 20,000 words of it a night, 14-hours'-worth, especially in the beginning. No sleep. "When I finished the second draft after two months, I looked at it - I was a physical wreck, of course, and I thought, 'I can't bear to write eight more drafts of this.' Because of course it takes 10 drafts to do anything decent in literature. Anyway, after two drafts I showed it to Ann Harris, and she said. 'Yes. It's great, but it needs a lot of work.' By the end there were 10 drafts and it took a year and a half. See, I get very restless if I can't do my own thing. And my own things are solitary, and that's one reason I like to live on my own.

"I'm very independent," she continues. "They only thing my mother ever told me was, 'If you've got any sense, you won't get married,' so I was never brain-washed into it. And to be honest, many women marry for a meal-ticket, you know . . . But I don't thate men. And I'm not a flag-waving feminist who ostentatiously carries copies of Ms about, or who refuses to wear underpants.

"I'm fat," she continues without stopping, "because I have a genuine thyroid deficiency that was not discovered until I was 32 . . . But the other day this lady journalist called me and really, she's much more attractive than me, and I told her, "I always have at least one boy friend tucked away,' and she said, 'Well that's more than I have . . .

"I get tired of having people under my feet, though. This getting-the-oldman's breakfast is not my cup of tea at all."

Well what about the old man getting her breakfast? It's the wrong thing to say. McCullough looks thoroughly appalled."

"Not in my kitchen, Mate."

Although she says she is "one of life's great enjoyers," a happy person, really, there is also a dark side to McCullough, and it emerges when she speaks of her dead father, who was a builde rin Australia, a man she calls without hesitation "an unmitigated bastard."

"He died too easily," she says wryly. "He died in his sleep. He should have died of cancer of the tongue."

And if that sounds bold and shocking to you, possibly more than you wanted to hear, it is partly for this reason: in many ways Australia is the Texas of the East, its expanses fostering both energy and enigres, its inhabitants never quite having resolved their status or relative newness. Colleen McCullough found it "intellectually stifling there." But she also never quite got over it.

"I don't know," she says. "I think it's a poor existence in many ways. Especially for a woman."

Whether or not enough Americans will want to plunge into a literary excorcism of that Continent to justify Harper & Row's and Avon's hopes, is another question. In any cae McCullough can't seem to get the place out of her system: Her next novel will also be on Australia. For all that, she left it in 1963 to go to England. For all that, she wants to continue to live here in the States. In Gulford, Conn., to be exact, becaue "I like it there."

And for all that she says she'll probably continue to be a neurophysiological technician. She likes the job. She also liked the job she had as a typist last winter for a bare few months in Cambridge, England, before she came back here.

"I'll probably live pretty much as I always have," she maintains. Well, since she's getting half of that $19 million (before taxes, before taking out the agent's fee), since there might be a motion picture possibly (agent Swifty Lazar is handling that), his many sound doubtful to you. After all the acclaim, and the interviews, and the parties that may, in fact, sound very doubtful.

And yet - you look at Colleen McClulough, almost 40, very frank, and relentlessly Australian, and you think: Well maybe it will work out all right for her. Maybe she'll survive all this craziness.

Ask her, for instance, where she learned to write, and she'll raise her long thick arm. Then she adds, "Well I did do The Famous Writers course. Back in '69, it was. And if I remember correctly, I got pretty good grades . . ."