DETAINEES who protested on the roof of a Sydney detention centre this week have been put in maximum security isolation as punishment, a refugee advocate says.

Nine Chinese nationals, including five men and four women, one of whom is two months pregnant, climbed on to the roof of the Villawood centre on Wednesday morning.

One of them came down at 4pm (AEST) yesterday and the rest followed about six hours later.

Another 11 asylum seekers ended a 30-hour protest on the roof on Tuesday night.

Justice Action Network spokesman Jamal Daoud says he believes all but one of the rooftop protesters are now in maximum security compounds.

"All of them are in Stage One at the moment, except the pregnant woman - she was taken to hospital last night," said Mr Daoud, who has been in regular contact with detainees.

"They usually put someone in there for punishment.... They are in isolation, they are not allowed to go outside the room."

Serco, the company that runs the centre, declined to comment on the asylum seekers' situation and said all inquiries should be referred to the Department of Immigration and Citizenship (DIAC).

A DIAC spokesman refused to comment on where the protesters were housed, saying it was a matter for Serco.

"As far as where they're housed, that's a matter for Serco and their ongoing management of the centre," he said.

The spokesman said four of the nine Chinese detainees had been hospitalised on Thursday night and two remained in hospital.

"The four of them were taken to hospital last night as a precautionary measure," the spokesman said.

"Two have been taken back to Villawood and two remained in hospital under observation. My understanding is that they are still there at this point."

The nine detainees are believed to be from China's Fujian province and have been in Australia for between two and six months, entering the country on student and tourist visas.

Meanwhile, another group of 16 detainees are continuing a hunger strike, which is believed to be in its sixth or seventh day.

A spokesman for DIAC said five of them were hospitalised on Wednesday and four were returned to Villawood yesterday.

He said three of them had been seen eating.

But Mr Daoud claims none of them have accepted food.

"The others are very, very weak - I talked to one of them yesterday and he was very, very weak, he barely could talk and he was distressed too," he said.

"Yesterday there was negotiations (with DIAC) ... but there was no positive outcome."

