Australia II's remarkable streak of bad fortune continued today as she raced to the verge of an America's Cup victory over Liberty, only to have the victory stripped by a vanishing wind.

"It's a murrain," said one Australian, citing his country's name for a pox of the kind that leaves its victims hairless and covered with boils. "It's the curse of Rhode Island Sound."

Australia II still trails, 2-0, in the best-of-seven series for yachting's sweetest prize after back-to-back gear breakdowns apparently cost her leads in the first two races. She led Liberty today by almost six minutes--a light-year in 12-meter competition--after five legs of racing in breezes that started moderate and fell to nearly nil.

But her turn for the 4 1/2-mile final leg to decisive victory was an exercise in futility. By cup rules a race is abandoned (and thus must be reraced) if it is not completed in 5 hours 15 minutes.

There was only an hour left for her to finish, and with the wind dropping to a mere whisper she had no chance.

Australia II was halfway up the glassy course, with Liberty slatting along in the deep distance a half-mile behind, when the message from the race committee yacht Black Knight came by radio: "The time limit has expired. The race has been abandoned."

So Australia, which proved today it will be extremely difficult to compete with in anything less than vigorous winds, must try again Sunday to break Liberty's shutout in this 25th defense of the cup the United States has never lost.

Foreign challengers have managed to win but three races here since the cup competition switched to 12-meter yachts in 1958.

The greatest winning margin for any challenger was 1 minute 2 seconds by Gretel II over Intrepid in 1970.

Today, Australia II was on the way to breaking that record as skipper John Bertrand beat Liberty's Dennis Conner to the starting line by 11 seconds, then expanded the lead to 1 minute 15 seconds by the first mark 4 1/2 miles upwind.

Bertrand seemed to be operating on strict orders today to answer every tactical move Conner made.

In both of the first two races he had allowed Liberty to slip off on her own when Australia had the early lead, and both times the wind-wise Conner caught shifts and used them to retake the lead.

This time Bertrand tacked through the breeze every time Conner did in the shifting southerly breeze and it initially served him well.

He expanded the lead to 2 minutes at the second mark, then recorded leads of 1:58, 1:46 and an astonishing 5:57 at the final turn as the wind petered out.

But his insistence on answering every Conner move may have hurt Bertrand's cause.

On the fourth leg, with the wind--which started the day at 10 knots--beginning to fade, it became apparent that time might be a factor.

Conner, struggling along well behind Australia, elected to sail far off course and into the spectator fleet wash. Bertrand followed, and it cost him time.

Conner admitted that by then he was thinking about the time limit and using tactics accordingly. "We figured we had to average 4.6 knots to finish in time and we were doing 3.6. It looked like the limit would be a factor and we weren't upset to be off on one side of the course," said Conner.

But few observers felt the race could have been finished in any event.

Bertrand, asked if he felt the time limit was unfair, laughed and said, "It should be expanded by an hour, of course."

Is he beginning to feel snakebit? "It was unfortunate, but all we can do is go out every day and do the best we can. There is no other philosophy you can pursue."

Toward the end today, Bertrand sent a crewman up the mast, but this time it wasn't to fix anything as it had been in Race 2, when his mainsail ripped at the head.

This time, said Bertrand, "He was looking for wind."

It was a touching sight--the lookout high on the 90-foot spar, searching the calm waters for relief, somewhere.

Meantime Conner was quizzing his navigator.

"We had a constant update from Halsey Herreshoff," said Liberty's skipper.

"When it got to the point where they would have to do about 1,000 miles an hour to finish, we knew we were in," Herreshoff said.

As to his assessment of Australia's evident speed advantage today, Conner said, "In this kind of (moderate) breeze Australia II has gained about 1 minute on each weather (upwind) leg against most conventional 12 meters. She did the same to us today."

Added Conner, "There was nothing wrong with our boat except it doesn't seem as fast as Australia II."

More moderate weather is expected Sunday, with a forecast of 10- to 15-knot winds from the west or south.