The five, who were not identified, were arrested after towing a 7m motor boat almost 3000km from Melbourne to Cairns.
In an unrelated incident on the same day, police in Sydney arrested an 18-year-old man for allegedly planning to obtain a gun to commit a terrorist act targeting public buildings or landmarks in the city.
Australia, a staunch US ally, has been on heightened alert for attacks by home-grown radicals since 2014 and authorities say they have thwarted a number of plots.
There have been several lone-wolf assaults, including a 2014 cafe siege in Sydney that left two hostages and the gunman dead.
About 100 people have left Australia for Syria to fight alongside organisations such as Isis, Australia's Immigration Minister said last month.
Also in 2014, police shot dead a Melbourne teenager after he stabbed two counter-terrorism officers.
A 15-year-old boy fired on an accountant at police headquarters in a Sydney suburb last October and was then killed in a gunfight with police.
- Reuters