Violent protest at Abbott's Sydney event

Violent scenes have erupted at a Sydney fundraising event for Tony Abbott, with protesters clashing with police and guests, including the former prime minister's sister.

Several hundred protesters outside the event in Redfern confronted invited guests, including Mr Abbott's sister Christine Forster, whose jacket was ripped as she struggled through the crowd.

Ms Forster was forced back from the entrance until police formed a ring around her and pushed their way through the crowd.

The protest is being held to oppose Australia's offshore detention system and was attended by Immigration Minister Peter Dutton.

However, both Mr Dutton or Mr Abbott eluded the protesters and were not seen entering the event.

One protester was pushed over by a man arriving at the event, before police were able to intervene.

Another guest, who did not want to be named, told AAP he believed the protesters were "doing more harm than good for their course".

NSW police confirmed at least three people had been arrested during the protest, but could not yet confirm the charges.

Greens Senator Lee Rhiannon, who was at the protest, defended the protesters and said the refugees on Manus Island needed to be evacuated.

"What was full-on was the police, the police have mishandled tonight," Ms Rhiannon told AAP.

"We're here standing with the 600 men on Manus Island, I'm angry, so many people are angry."

Hundreds of men have been holed up in the Manus Island detention facility for more than a week after it was shut down.

A showdown is looming at the centre with PNG authorities warning they will use force to remove barricaded asylum seekers.

The men believe it's safer to remain in the compound than risk being attacked by locals at new facilities near the main township of Lorengau.