SYDNEY -- A United Nations team began investigating Australia's system of mandatory detention of illegal immigrants Tuesday at one of the country's most notorious centers.

Protesters outside the remote south Australian Woomera Detention Center warned the UN officials not to be fooled by the center's improved appearance. And they claimed asylum seekers were on a hunger strike.

Refugee activist Matthew Hamon, who visited the center Tuesday, said trees had been planted and the dormitories repainted before the UN visit.

"Whatever the UN saw today, it's not really an accurate representation of what the Detention Center has been like for the past two or three years," Hamon said.

"I'm told by detainees that flowers were given to them to plant, there was painting done and the razor wire was actually removed from certain areas to make it look less formidable," he added.

Ross Parry, who mans a refugee protest camp outside the center, said the detainees had told him they were on hunger strike.

"They've been in there for months, some of them have been in there for years and they are very desperate people," he added.

A spokesman for Immigration Minister Philip Ruddock denied there was a hunger strike.

"Nobody at the management center knows about it and people are still turning up for meals," he told AFP.

However, he did agree maintenance work had improved the center.

"This includes repairing substantial damage caused by detainees on a number of occasions at Woomera and other centers," he said.

"Any work undertaken was simply part of this process and not a reaction to the UN visit." The department confirmed razor wire had been removed from some internal fences at the center saying it had initially been installed as a temporary measure.

More than 1,000 Afghans have been offered 2,000 dollars (1,100 us) each, capped at 10,000 dollars per family, to return to Afghanistan, which Canberra says is now safe for their return.

The cash offer is part of the government's push to reduce its detention center population by encouraging Afghans trapped in the processing centers or fighting deportation to go home.

Many are in the Woomera compound, which was visited Tuesday by an Envoy of UN High Commissioner for Human Rights Mary Robinson.

The envoy, Indian Judge Prafullachandra Natwarlal Bhagwati, was to spend the day interviewing asylum seekers at Woomera, which has seen months of rioting, arson and self-mutilation by detainees. Many are what Canberra calls "rejectees" -- failed asylum seekers.

Bhagwati spoke to Jeremy Moore, a lawyer for detainees at Woomera, Monday night.

"It's incredibly important that Australia be under the spotlight of the world and that people can see exactly what is happening in Australia," Moore said.

"I'm expecting the UN to be scathing of how we treat refugees in this country." Bhagwati's arrival coincided with a visit by the UN working group of arbitrary detention, headed by Frenchman Louis Joinet, which will visit centers in western Australia, south Australia, Victoria and New South Wales.

Findings from the trip will be reported to the 59th session of the Commission on Human Rights next year.

The inspections come as Australia steps up efforts to repatriate illegal immigrants and Afghans who no longer qualify for asylum after the fall of the Taleban regime.

The UN team has declined to talk to media.