Seven detainees were on the roof of Sydney's Villawood Detention Centre at lunchtime today (NZT) after three more joined the protest as authorities worked to regain control at the centre.

The detainees sat beside a large white sign that read in black letters "We need help".

Riot police were seen to enter the building shortly after 9.30am.

Protests at the centre were triggered after two men climbed onto the roof early yesterday.

They were soon joined by 11 others and by midnight up to 100 people were involved, vandalising and setting fire to buildings.

Protesters torched an oxygen cylinder leading to an explosion shortly after 2am today.

Nine demountable buildings - medical centre, kitchen, dining room, laundry and a computer room - have been gutted by fire and the centre appeared a charred and mangled mess in the morning light.

Black smoke curled into the air above the centre at 9am, even though NSW Fire and Rescue crews and NSW Police had brought the blaze under control two hours earlier.

Police reportedly protected the firefighters as they battled the blaze from such things as hurled tiles.

"It's a criminal matter and they've done a wonderful job protecting the firefighters as they've done their job," immigration department spokesman Sandi Logan told reporters, adding an investigation would take place.

"There may well be charges laid from this investigation."

Mr Logan said it was an "ongoing challenge" to maintain the mental and physical health of the detainees.

"From time to time, frankly quite unacceptable, quite appalling non-compliant behaviours have occurred and this is an example of it overnight."

About 400 detainees are currently housed at the detention centre.

Mr Logan could not confirm reports the men were protesting because their visas had been rejected.

"But any suggestion that they're not being informed of the progress of their claim is nonsense ...

"I don't know the motivation.

"But it's clearly not going to help, in terms of endearing their settlement in Australia."

The detainees still on the roof are reportedly calling for a meeting with the immigration department but Mr Logan said case managers would continue to be their first point of contact.

"Until they come down, we won't be negotiating, but we are working and managing to get them down from the roof," he said.

Dr Graham Thom, refugee co-ordinator for Amnesty International Australia, said it was time for an investigation into detention centres because the situation inside them was going "from bad to worse".

"We really need to have a look at why are we detaining such large numbers of people in such remote places, and is this the best way for Australia to meet its obligations to protect people," he said.

Dr Thom said he was aware of people were becoming increasingly depressed at Villawood.

"We don't need to have these large detention centres where we keep people for a year and a half, if not more, until we finally break them," he said.

"When you put a group of men together who are extremely vulnerable ... this is the sort of psychological pressure that just builds up on these people."

Dr Thom also raised questions about the security officers at the detention centre who had their training in prisons.

"Do you have the right people with the right skills doing that job and providing the relevant activities and other things that they need?

"When you've come from a particular background with a particular skill set to suddenly get thrown into that environment, we do have concerns about whether the training is adequate."

- AAP