I think that it's harder to establish a fair price for a boat compared to a house or car, but here are a few tips, using some of the same ideas as for houses and cars, with a twist. I'm no expert, but here are some of the issues from our search, and from our friends' sales.
First, because you are in Australia, the origin and owner of the boat can be an important clue. For a foreigner owning the boat and who will be leaving Australia, look to the price of comparable boats in their home country. Not necessarily the same boat, considering one-offs, but size, age, hull material, and importantly, who designed the boat and who built it. Home built boats, no matter how much you love it, should go for a lower price than one from an established yard.
How much would it cost to repatriate the boat? Deduct that from the asking price and see what they say.
Designer - established reputation, higher price than an unknown.
Yard - same
How old are the electronics? Generally, here in the States we're told to discount their value since most people would want to put in their own.
There are so many websites for boats for sale, with multiple search criteria, you should be able to find lots of comparable boats to look at.
If you really like a boat, you can ask for a sea trial before making a final offer, but the broker will guide you a bit on this.
Survey is for after the offer to purchase is accepted and signed, with a contingent to survey clause in it. You might want to interview the surveyors to find out how familiar they are with the particular make of boat he will be surveying. The survey should point out any problems (osmosis, damage, etc.) and provide an estimated fair market value for the boat. If it's lower than your offer price, there are usually further negotiations. If it's higher, you can pat yourself on the back.
Maybe Canukiwis will check in and give you more ideas since they have recently gone through the exercise themselves.