As a protest enters its fourth day at Sydney's Villawood detention centre, police are reportedly no longer allowing food to be thrown to the four asylum seekers still on a roof at the facility.

A witness at the scene also says ambulance workers have stopped supplying the men with food and water, while guards have been using loud speakers overnight to keep the men awake.

Twenty-two detainees were yesterday taken to Silverwater Prison in Sydney's west for questioning over this week's riot.

Police say they have not yet made any arrests or laid charges.

The Australian Federal Police says its emphasis is on the security and safety of the protesters and that no further comment is appropriate.

The Immigration Department says Villawood Detention Centre is calm despite the ongoing rooftop protest and denies the centre is in lockdown.

A spokeswoman says the department is working hard to restore key services to detainees.

Ian Rintoul, from the Refugee Action Coalition, says he is concerned that the men being held in prison over the riot are not being made aware of their rights.

"In many of the countries they come from [people are] seized by the police and placed in jail in places where they have no legal rights," he said.

"They do not necessarily understand that they don't have to give a statement. They don't have to talk to the police.

"We are very concerned that no statements are made and there are no interviews with the police until they understand their rights."

The men are believed to be protesting against their failed visa applications.

The protest reached its peak when a small protest on Wednesday afternoon developed into a full-scale riot in which close to 100 asylum seekers burned down nine buildings at the centre.

A computer room, kitchen and medical centre were destroyed by fire and a large gas cylinder also exploded.

A former Villawood guard told ABC Television's Lateline program on Thursday that problems at the Sydney centre have been building for some time and he is not surprised about the riots.

In a statement, Serco, the private operator of detention centres in Australia, acknowledged an increased number of arrivals and longer periods of detention have placed significant pressures on their operations.

The company said it has provided additional training to staff beyond contractual requirements and has invested $1.5 million in staff training.

Villawood is the second Australian immigration detention centre to be set on fire this year.

A Federal Government-ordered review into riots at the Christmas Island centre in March, which saw tear gas and bean bag rounds being fired at asylum seekers, will now also investigate the Villawood protests.