Alexandra Rickham and Niki Birrell won Great Britain's first medal at the Para World Sailing Championships after taking silver in the SKUD.
The former world champions, who have had equipment issues in Melbourne, cannot be caught by third-placed Italy.
Defending world champions Liesl Tesch and Dan Fitzgibbon took gold with a day to spare in front of their home fans.
"I think realistically this morning we knew that [silver] was the best that could happen," said Birrell.
"We sailed over some weed on day two, and then we had a problem with Alex's canting seat.
"We have to respect the fact that [the Australians] sailed really well this week and deserve their world title.
"I think the scores would have been a lot closer had it not been for the seat issue and the weed. That being said, they sailed really well and we're going to have to find some more."