Victoria Police successfully applied for an interim preventative detention order (PDO), which allows the man to be detained without charge for up to 14 days.

It is the first time police in Victoria have used a PDO under the Terrorism (Community Protection) Act 2003, and has prompted concerns the man's detention could  trigger community backlash.

Five men, all aged 18 and 19, were arrested in Saturday's pre-dawn raids in Narre Warren, Hampton Park, Hallam and Eumemmerring.

It is alleged a terrorist attack targeting police officers had been planned for Anzac Day commemorations in Melbourne next weekend.

Hallam man Sevdet Besim, 18, has been charged with conspiring to commit a terrorist act.

Three other men, all from Narre Warren, were released pending further investigations but police expect to charge one on summons with prohibited weapons offences.

Police have not named the 18-year-old Hampton Park man currently in custody.

A PDO is granted in order to preserve evidence relating to a recent or imminent terrorist act, or to prevent an imminent terrorist act from occurring.

The orders form part of a swathe of anti-terrorism powers granted a decade ago but, according to terror expert Professor Greg Barton, they were not used until last year.

As part of Operation Appleby, police successfully obtained the orders for three men following counter-terrorism raids conducted in Sydney and Brisbane last September. All three were later released without charge.

Professor Barton, from Monash University's Global Terrorism Research Centre, said the orders had "very limited utility" and were "not particularly useful" in police investigations.

"Any information you get from somebody when they're in detention cannot be used in any legal proceedings," Professor Barton said.

"All you can do with an interim order is keep them from getting in the way of the current investigation."

Professor Barton believes it is unlikely the man will be charged.

"If they had reasonable grounds for charging him they would have," he said. "I think [the order] is to try and stop him from muddying the waters."

The order may have been granted to prevent the man from communicating with a particular peer network, Professor Barton said.

But detaining someone without charge risks community backlash, which can negatively affect the relationship police have with community and social groups they rely on for information or assistance.

"It risks relations [police] have been building up with community groups," Professor Barton said.

The state government's security and emergency management cabinet met on Saturday night and were briefed by police on the terror plot.

Premier Daniel Andrews said he had spoken to religious and community leaders in the wake of the terror raids.

He defended the use of the interim orders placed on one of the accused, saying authorities needed the best tools to carry out their job of ensuring public safety.

But he would not be drawn on why only one of the five accused was being slapped with the detention order.

With Richard Willingham