Hi Martin,
Not trying to scare you, but be aware that there are marine stingers in Queensland waters everywhere north of about Gladstone and they are active from November until around April. You might want to take that into consideration when planning your journey.
some links:
http://www.health.qld.gov.au/goodhealthintnq/topics/jellyfish.aspwww.marinestingers.com.au/safety/Otherwise enjoy your trip, its awesome up there in winter