There's nothing like a mighty House vote on the budget and a presidential address to ruffle even the best of Washington's parties.

Budget Director David A. Stockman didn't quite make it to the Australian Embassy last night. But presidential counselor Fred Fielding and White House communications director David Gergen came for the steak tartare and slipped out before the screening of "The Thorn Birds." And U.S. Information Agency director Charles Z. Wick sat through the entire show, but arranged to pick up a tape of President Reagan's address the moment it was over.

"A winner!" Wick told Tony Thomopoulos, president of ABC Entertainment, after the screening.

"My wife and I grew up in that part of Australia and we also experienced the distressing poverty of the farm community," said Australian Ambassador Sir Robert Cotton. "I can vouch for its accuracy."

"It's an Australian 'Dynasty,' " said Nancy Clark Reynolds. "But I missed the Australian accents."

Last night ABC sandwiched 10 hours of "The Thorn Birds," a historical family saga based on Colleen McCullough's best-selling novel, into a 56-minute preview for Washington's politicians. The ABC "novel for television" will run this Sunday through March 30 despite complaints from the Roman Catholic Church that the film, which depicts a priest who breaks his vow of celibacy, is in poor taste for Holy Week.

The Catholic Conference has called the movie "an affront to good taste and religious sensibilities." McDonald's, the fast-food company, had asked its franchises not to advertise during the show unless their ads were placed early in the series--before the priest breaks his vow.

"We have sold 100 percent of advertising for the show," said Thomopoulos. "I don't know what the McDonald's thing is all about, but I imagine it's resolved by now."

Others crowding the buffet tables included CIA chief William Casey and U.S. Chief of Protocol Selwa Roosevelt. Those from Washington's power structure who couldn't break away from Capitol Hill, where the Democratic budget was being approved, or from the White House, where President Reagan was calling for a new round of arms development, were represented.

"I'm here because Colleen McCullough is a great friend of mine," said Nancy Thurmond, wife of Sen. Strom Thurmond (R-S.C.).

"You know," continued Thurmond, "she used to live in Connecticut, but all the fame left her no privacy. So now she's moved to this tiny island off the coast of Australia. We haven't talked in a few months."

"Jim called and said he couldn't make it because of the president's dinner before the televised address ," said Nancy Rosebush, wife of James Rosebush, the first lady's chief of staff. "But I said I was coming anyway. I couldn't put the book down . . . I don't know if there's time to see the whole series."

In the end, when the last bite of kiwi had disappeared from silver platters, each guest was given a little reminder to take home--a gold kangaroo pin.