The Sydney Opera House is no stranger to drama, but today its famous sails became the stage.

New South Wales Police have charged five activists who scaled the tourist hot-spot holding banners complaining about the treatment of refugees on Manus Island.

The Whistleblowers, Activists and Citizens Alliance (WACA) said it was behind the stunt, which began about 10.30am.

Activists managed to climb the icon despite a recent security upgrade, designed to bring the venue up to "international" standard.

A NSW Police officer speaks to a protester on top of the Sydney Opera House. ( Twitter: @akaWACA )

The Opera House's postcard-worthy location at Sydney's heaving tourism and water transport hub of Circular Quay, means it is considered a target for demonstrations and terrorism.

NSW Police said officers from the Police Rescue and Bomb Disposal Unit climbed up the sails of the Opera House, where they arrested two men, aged 23 and 26, and three women, one aged 25 and two aged 27.

They were taken to Day Street Police Station and charged with trespass. No-one was injured.

They were granted conditional bail and issued with place restrictions not to enter the Sydney CBD.

The Manus Island Regional Processing Centre — used to screen people seeking asylum in Australia — officially closed on October 31.

About 600 refugees have remained in the centre, which has had its services cut off.

WACA said the protest was in response to the "humanitarian crisis" unfolding on the Island.

Protesters can be seen walking up the sails of the Sydney Opera House. ( Twitter: @akaWACA )

Police appeared to thwart an attempt to unfurl a larger banner, on the west-facing sail facing the Harbour Bridge.

A spokeswoman for the group said "a lot of preparation" had gone into the protest.

In a video posted to social media, one of the protestors on top of the Opera House called on the Turnbull Government and Labor to evacuate Manus Island and bring refugees to Australia.

Police told three protestors on the ground — two women and a man — to move on.