The counter-terrorism chief of the Australian Federal Police has revealed more Islamic State (IS) recruits have managed to leave Australia "a few days ago" without being detected by authorities.

AFP Assistant Commissioner Neil Gaughan made the revelation while appearing before a Parliamentary Committee examining new counter-terrorism legislation, known as the Foreign Fighters Bill.

The Assistant Commissioner told the committee Australia's control order regime needs to be overhauled with a lower threshold for evidence, so police can catch terror suspects.

"I think what we're missing is the ability to stop people – the enablers and the supporters. We haven't got anything there," Assistant Commissioner Gaughan warned.

He said more Australians have managed to slip out of the country to join IS fighters in the Middle East.

"There are some people who travelled a few days ago that were not on anyone's radar," he said.

"We got wind of it after the fact but the fact is there are still people travelling.

"And regardless of what we're doing, we're not stopping that, so we need some other tools."

In August, counter terrorism units were established in all major airports across the country.

The units consist of an additional 80 border force officers and are stationed at international airports to monitor the movements of people on the Government's national security watch lists.

The controversial Foreign Fighters Bill was passed last month.

The new laws, which are designed to stop Australians fighting in overseas conflicts, make it easier for the Government to cancel passports and allow authorities to declare some conflicts as "no go" zones for Australian travellers.

Assistant Commissioner Gaughan said the greater powers were needed to stop those facilitating and supporting home-grown extremists.

He also revealed a current operation into people who authorities believe are planning a terrorist attack but said the AFP lacked the powers to detain them.

"We've been able to stop that attack at this stage but the threat still remains because the majority of the people are still out in the public domain," he said.

"There are, I would say, a handful of facilitation groups operating up and down the east coast [of Australia] that at the moment are just far enough away from law enforcement that we can't arrest them.

"Now if we had sufficient evidence we would arrest [them] – we have done that on a couple of occasions."

Since mid-October, there have been three unconfirmed reports of Australian IS fighters being killed in Syria, including the Western Sydney actor turned Islamic State recruiter Mohammad Ali Baryalei.

Baryalei had been accused of masterminding a plot to kill random members of the public in Sydney and Brisbane, and was the catalyst for September's unprecedented counter-terrorism raids, when more than 800 police raided homes in Sydney and Brisbane and charged four people.

He was said to hold a trusted position in IS's operational command and to have facilitated the recruitment of at least half of the 60 Australians currently fighting in the Middle East.

Consular assistance is no longer available in Syria or Iraq and the Government is warning against travel to the two countries.