Sydney: The government will next week introduce legislation in parliament to tackle a growing terror threat, the attorney general said on Saturday, in the aftermath of Australia’s biggest crackdown in history.

George Brandis announced the National Security Legislation Amendment Bill would go before the Senate on Wednesday.

“These bills will give our national security agencies and the Australian federal police stronger powers, but there will also be stronger protections,” he said.

“The government has gone about this task of re-writing Australia’s national security legislation for the first time in a generation... to ensure that the agencies have all the powers that they need to keep Australians safe without in anyway diminishing the oversight by parliament and... without in anyway diminishing protections.”

Brandis did not disclose the extent of the new powers or oversights but said there was bi-partisan support for the legislation and he expected the “agency powers” bill to clear parliament within a fortnight and the “foreign fighter” bill in the coming weeks.

Thursday’s unprecedented raids in Sydney and Brisbane foiled a plot by Islamic State of Iraq and the Levant militants to carry out gruesome “demonstration executions” that could have taken place within days, Prime Minister Tony Abbott said.

Justice Minister Michael Keenan explained that the new laws would “modernise” existing legislation.

“The threat of the random act of violence that was acted upon on Thursday’s raids is obviously quite different to the sorts of traditional terrorist activity that we might have been targeting,” he said.

“We need to make sure that we’ve got a regime in Australia that’s modern and flexible.”

Security has been stepped up across the country after parliament and government officials had been mentioned as potential targets in “chatter” between extremist networks in the Middle East and Australia.

Fifteen people were arrested when hundreds of police officers raided dozens of homes in Sydney and Brisbane on Thursday, but only one person remained in custody on Saturday, officials said.

Omarjan Azari, 22, was charged with planning a terrorist act that prosecutors said was intended to “shock, horrify and terrify” the community and involved the “random selection of persons to rather gruesomely execute” on camera.

Federal police had for the first time used preventive detention orders to hold three of the 15 without charge, the Australian Broadcasting Corporation reported. The men were released on Friday.

The orders are designed to counter an imminent threat of attack and can be used to hold people for as long as 14 days.

But Abbott says current legislation is inadequate to fight the threats to Australia from groups such as Isil, which he has described as the nation’s greatest national security challenge.

Under the new powers, advocating a terrorist act will become illegal, The Weekend Australian reported.

The offence will carry a maximum five-year jail sentence and make it illegal for an individual to intentionally counsel, promote, encourage or urge a terrorist act, the newspaper said.

Police will also be given powers to secretly search the homes of suspects.

The government will further seek powers to proscribe visits to cities or regions where terror groups are active. People travelling to such areas without a valid reason could face prosecution.

The government believes up to 60 Australians are fighting alongside Isil militants, while another 100 are actively working to support the movement at home.

Canberra has committed 600 troops and aircraft to the US-led coalition gearing up to destroy Isil in Iraq.