The link to the AQIS (Australian Quarantine and Inspection Service) page on entering Australia is here
I'd strongly advise browsing the entire site, and following the links there to the Customs
and Immigration sites.
Without knowing the full picture you must report well before you arrive to AQUIS and Immigration. Yachts have had very unpleasant experiences when they did not follow requirements.
You must check the section on "Biofouling". My recollection (unreliable, hazy, getting old) is that yachts can antifoul at the last place they visit before Australia
, but your questions are better directed to AQUIS and Immigration (by email