For Australian passport holders entry into Fiji is visa free and four month stay allowed and for French Polynesia also no visa required and allowed to stay for ninety days.
Generally you must have at least six months remaining on a passport after the date of entry in a country.

Inoculations are no longer required for most countries but you will need a valid Covid certificate to enter most countries including coming back to Australia. Probably you will need to do a RAT or PCR test before being allowed into the country

To leave you don't need to see anyone other than getting the required exit stamp in your passport which happens when you check out of Australia.

To my knowledge Once boat and all crew are cleared to leave you all get yr passports stamped at approved customs office then you have 24hrs to set sail, or you have to explain why the delay.

I don't have personal experience of sailing overseas: only have read about it.
If you are crew, the captain will be responsible for the ship and all the passengers aboard.
The captain would ensure the paperwork, that the ship is registered, and cleared to leave one country. He would arrange with Customs (customs inspection), Immigration/Border Force. I understand that ships entering Aust need to give at least 48 hrs notice of entering. The captain will need copies of all passports, passport numbers, health insurance, etc.
As crew, you won't need to worry about all those things; mostly your health & certificates, insurance, etc.