As many workers were enjoying their lunch or exercising in the CBD at 1pm, a skywriter was penning the words "Shut down Manus" above the iconic Opera House.

The message was to mark the one-year anniversary when asylum seeker Reza Barati was killed when violence swept through the centre on February 17, 2014.

It came one week after the Australian Human Rights Commission released its report into children in detention, saying hundreds of children had been abused and calling for them to be removed from detention immediately.

The report sparked fierce debate, with Prime Minister Tony Abbott criticising the commission over the report, saying it should be "ashamed of itself" and that the report was a blatant attack on his government.

Three artists, who are not associated with any refugee or political groups, crowd-funded the idea for the skywriting and raised $9700 online in a week.

One of the artists, Asha Bee-Abraham, said the trio had already been contacted by an asylum seeker on Manus Island thanking them for their efforts.

"We got a Facebook message from a detainee saying that 'even words cannot express my appreciation to you and all your people who have empathy for our plight I am so happy to see that lots of people with hearts and compassion still exist'.

"It's fantastic, that's exactly the sort of impact we wanted to have," she said. "So many Australians don't agree with the government's policy and want to treat asylum seekers with dignity."

The group also commissioned a skywriter to write "Close Nauru" over Parliament House in Canberra at 3pm.

"There can be no excuse, children and men are suffering in the detention centres," Ms Bee-Abraham said.

There are now 136 children in mainland Australian detention facilities and 116 children in Nauru.

Last month asylum seekers on Manus Island staged protests and hunger strikes, saying they would killed if they were resettled in the community in Papua New Guinea.

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