A magistrate has granted police another six days to hold in detention five men arrested over an alleged plot to leave Australia by boat and join ISIL in the Middle East.

The men, including radical notorious Islamic preacher Musa Cerantonio, were arrested in Cairns on Tuesday night.

Police allege the group had towed a seven-metre fibreglass boat from Melbourne and had hoped to use it to travel from Cape York to Indonesia, from which they would then make their way to Syria.

The Australian Federal Police this afternoon confirmed a magistrate had granted authorities the extended period of detention under “specified times” provisions.

“These detention provisions recognise that terrorism investigations can be complex and that there may be legitimate reasons for extended periods of detention for suspects in such matters,” the AFP said in a statement.

“The use of ‘specified time’ provisions during investigations like this is vital to gathering, assessing and compiling relevant evidence.

“It should be noted that the presumption of innocence still applies to these men, and no criminal charges have been laid.”

Musa Cerantonio.

The provisions state the extended period of detention must not exceed seven days.

After the arrests, police confirmed the men are considered to be extremists, and their passports had already been cancelled.

Mr Cerantonio was deported from the Philippines in 2014, accused of being a radical preacher promoting ISIL.

His Australian passport was cancelled upon his arrival back in Australia.

Mr Cerantonio, a Melbourne-raised former Catholic with Italian heritage, changed his name from Robert after converting to Islam aged 17.

Another of the men is believed to Western Australian Shayden Thorne, who was jailed in Saudi Arabia for possessing terror-related literature.

Mr Thorne is the brother of preacher Junaid Thorne, who was jailed for eight months last year after booking flights under a fake name.

ASIO figures show at least 177 Australians have had their passports cancelled due to concerns they might leave the country to join the terror group in the Middle East.