I did the 3,800 mile trip from Hervey Bay, Qld to Perth last year in a 20 metre fast cat. I was really looking forward to the experience but it was a financial nightmare (cost me $138,000 when I had budgeted for 75K !) and I can't say it gave me a lot of pleasure either.
You can read some of the stuff that I wrote at the time at www.spiritofmindarie.com.au
There was quite a lot of stuff that came out afterwards as well !
When you get up onto the northern coast, fuel will be your overriding concern and it's very desolate country. Darwin to Broome is 673 miles with nothing sensible in between.
At the risk of sounding patronising, a circumnavigation in a rib is only for the serious adventurer. Sunrider, I take my hat off to you sir. I dread to think what the handling characteristics of a 7.3m rib would be with 3,000 litres of fuel aboard !