Hi mate , average cost on the back of a truck will set you back 4-5k for a 6m boat.
Google boat transport there is a web site where you can load in your boat details length , weight ect and you will be sent quotes from different operators including privateers that will tow your boat over , beware that their insurance is comprehensive.
I was patient and waited for my tub to pop up for sale here in WA , although I did have to drive to Geraldton to pick it up.
good luck

 I looked at transport for a vehicle recently and that was by ship to Freo, was a lot less than 4K. Can't recall the details but do a search via shipping from Sydney to Fremantle.

 Wow, I got a car from Tassie to Perth for $1700, I figured it would be around the same.

 I put my Boat on a car carrier from Melbourne to Perth. I Used McPhee transport here in perth. I payed $75/m3. It was hassle free and they called me when it was coming off the boat in freo, I picked her up from the gate.

I done a lot of calculations, from towing, trucking, trains, driving myself and seriously this was by far the best deal. I would highly recommend them. My boat was insured through Club marine, who insure you through the whole trip as long as your boat isn't lifted of the trailer.

6m Ali on trailer, full length about 7.4, 2.8 high, 2.5 wide, $3310

Good Luck

I paid less than all the above prices, cash, to get the 610 bar crusher shipped over from Syndey last year through Shane at TTS Australia http://www.ttsaustralia.com.au/

edit: also had club marine insurance who insured the whole way from the moment boat was delivered to transport depot