But indigenous organisers have vowed "we won't stop", setting up a potential clash with police officers on Saturday.

The police commissioner is seeking an injunction in the Supreme Court on the grounds the rally would breach COVID-19 health orders.

"All of us have given up so much and worked so hard to make sure we get on top of the virus," Premier Gladys Berejiklian told reporters on Friday.

"What this protest has turned into is a flagrant disregard of the health rules. We can't afford to have exceptions for anyone."

The Liberal leader argued the protest initially proposed by organisers was far smaller than that which was now scheduled.

She said she empathised with how people felt about the issue but asked them to express their views in a COVID safe manner, for example on Facebook.

Police Commissioner Mick Fuller said if the court action was successful and thousands of people turned up in contravention of any Supreme Court order "then obviously all of the police powers available to us can be used".

"But we would much rather see a peaceful outcome," he said.

Greens MP David Shoebridge told AAP: "Whatever the police say, thousands of people will attend tomorrow."

"What's needed now is cooperation, understanding and peacefully working together, not court orders and the implicit threat of more police violence," he said.

But the government says there only needs to be a few people in a large crowd with coronavirus to undo months of hard work limiting its spread.

"The potential for a second wave and an outbreak of coronavirus is extremely high in NSW," Ms Berejiklian warned.

Australia's chief medical officer on Friday said authorities had been terrified of any outbreak in indigenous communities with higher comorbidities.

"The risk to loss of life and spread of this virus in some of those remote communities would be catastrophic," Professor Brendan Murphy told reporters.

Prime Minister Scott Morrison on Friday had a clear message: "I say to them, don't go."

Under current restrictions in NSW, up to 20 people can attend weddings and up to 50 can go to funerals, places of worship, restaurants, pubs and cafes.

Householders are allowed up to five visitors, and outdoor gatherings are restricted to 10 people.

Saturday's rally in Sydney is being held to protest the deaths of Aboriginal people in custody and in solidarity with those outraged in the United States by the alleged murder of George Floyd by Minneapolis police.

Protest spokeswoman Faith Black said indigenous Australians were taking action while there was "worldwide recognition of how black people are being treated".

"We won't stop - we want people to look at us, hear us and stand with us," she said outside the Supreme Court.

The mother of David Dungay Jnr - who died in Sydney's Long Bay jail in late 2015 - said racism killed her son and his body was returned with "big monster hands" marked on his back.

"Nothing is stopping me - I don't care if they shoot me," Leetona Dungay said.

NSW Police Minister David Elliott earlier on Friday said anyone seeking to gather during a pandemic was "certifiably insane" and "nuts".

"If you attend a mass gathering and then expose any disease to a loved one, someone who is vulnerable, the elderly, you've acted completely inappropriately," Mr Elliott told 2GB Radio.

Four new COVID-19 cases were reported in NSW on Friday. All were returned travellers in hotel quarantine.

Australian Associated Press