A man accused of being involved in a plot to "gruesomely" execute members of the public in Australia has been refused bail in a Sydney court.

Omarjan Azari, 22, fronted Sydney Central Local Court after being arrested in a pre-dawn counter-terrorism raid in Sydney this morning.

He has been charged with one count of acting in preparation/planning of a terrorist attack. His lawyers did not apply for bail and it was formally refused.

It's believed he's alleged to have planned to kidnap a random member of the public, drape them in an Islamic State flag and film their beheading.

At least 14 others were arrested in the pre-dawn counter-terrorism raids across Sydney and Brisbane. It's not known if they too have been charged.

The raids were the largest in the nation's history, with more than 800 Australian police involved in Operation Appleby. Some of those detained are believed to have links to the terror group ISIL.

NSW Police said while the group was actively preparing there was no information about a "specific" attack, including dates and times.

Prime Minister Tony Abbott later confirmed authorities had acted on intelligence showing directions for the plot were coming from "an Australian who is quite high in ISIL (Islamic State in Iraq and the Levant)".

The raids were pre-emptive in nature, as The Australian Federal Police say a suspected terrorist cell "was close to an attack", News Ltd reported.

The arrests in Sydney follow months of surveillance of people linked to the terrorist group ISIL, which has been cutting a barbaric path through Iraq and Syria.

Australia's outgoing spy chief David Irvine last week raised the terror alert level from medium to high amid growing fears about the prosect of terror attack on home soil.

There are about 60 Australians believed to be fighting in Iraq and Syria with groups such as Islamic State, while another 100 are suspected of providing support from Australia.

Immigration Minister Scott Morrison said the massive operation was a reminder of the terrorist threat facing Australia.

"I think [it] demonstrates the very real threat that's there and the incredibly good work which is being done by our agencies, and I think again supports why the government has been so strong in its response to this threat," he said.

Brisbane raids not linked

Three raids in Queensland don't appear to be directly related to a mass counter-terrorism operation in NSW but investigations are continuing.

Australian Federal Police acting commissioner Andrew Colvin said the raids southeast of Brisbane this morning were certainly linked to similar raids in the area last week.

That operation resulted in two men, Omar Succarieh and Agim Kruezi, being charged with terrorism-related offences.

Mr Colvin said about 70 officers were involved in the Queensland raids.

He said the simultaneous interstate raids did not necessarily mean both operations were linked, but did show police were well co-ordinated across the country.

"While the raids in Queensland are not directly related to what's happened here today in NSW, the investigations continue, and we are looking at linkages between the two," Mr Colvin told reporters in Sydney.

Succarieh, 31, was last week charged with terrorism-related offences and is due to apply for bail in court today.

Succarieh stands accused of fundraising for Syria-based extremist group Jabhat al-Nusra and helping another man, Agim Kruezi, obtain funds to fight for a terror organisation overseas.

Kruezi, 22, has alleged links to the Islamic State group.