Australia II, the controversial America's Cup challenger that has been nearly unbeatable on Rhode Island Sound all summer, toyed with defeat in an error-strewn outing today before sweeping past Britain's Victory '83 for another convincing victory.

Skipper John Bertrand began badly, putting Australia II over the line before the starting gun for no discernible reason.

Bertrand, who said later it was simply a mistake, was required to recircle and cross the line again.

The gaffe put Australia II 45 seconds behind at the outset just after noon.

Her vaunted speed advantage was not enough to put her back on top until the bright sun was dipping toward the Rhode Island coast.

Nor did her mistakes end at the starting line on a day when the winds started out a brisk norther, then quit altogether before resuming from the south.

Trailing by 32 seconds after one-third of the race, Australia II's crew chose the wrong sail to counter a wind shift. Victory chose correctly and when the two yachts rounded the halfway mark, the dark blue British yacht was ahead by an astonishing 1 minute 46 seconds.

Victory syndicate chief Peter deSavary was observed all but crowing from his low-slung motor yacht, Lisanor.

He did not crow for long.

Australia II closed the margin to 17 seconds in the next leg, then with a building breeze in her teeth for the final nine miles of the 24.3-mile race she shot ahead, leading by 1:10 at the fifth mark and 1:54 at the finish.

The triumph put Australia II firmly in first place with a record of 3-0 in the semifinal series to select a challenger for the America's Cup races next month.

Since the start of racing in June she's lost only four times in 43 matches.

Victory '83, which won her first two semifinal series races when her opponents dropped out with gear trouble, fell to 2-1.

The Italian boat Azzurra improved her record to 1-2 by defeating 0-3 Canada by 8 minutes 53 seconds today.

The Canadians led for half of that race, but were stopped dead in the water by rigging troubles and never recovered.

Australia II sailed so well in the final stages today that she wound up the first of all four foreign contenders to cross the finish, even though Azzurra and Canada started 15 minutes ahead of her and ran the same course. Thus she went from dead last at the start to first by almost two minutes at the finish.

One potential cloud darkened the triumph. Victory hoisted a red protest flag halfway through the race, puzzling observers who saw no Australian transgressions.

Later, sources in the British camp said the protest was of Australia II's use of a sail from another Australian 12-meter, Challenge 12, which already has been eliminated from competition.

But the British said they were not sure whether they would press the protest, which veteran cup watchers gave little chance of success.

The three American yachts seeking selection as cup defender will return to racing Tuesday.

Pairings announced today pit Courageous and Defender in the first race of the final series, with Courageous and Liberty racing Wednesday and Defender and Liberty meeting Thursday.

The three boats will continue racing in that pattern until one is dismissed by the selection committee. An American defender and a foreign challenger must be named by Sept. 8, with the final series slated to begin Sept. 13.