Australia's most populous state scrapped 33,000 fines issued for breaking Covid-19 rules Tuesday, after a top court ruled they were invalid.

New South Wales authorities were forced to retract more than half their pandemic-era fines, which were found to be too vague in describing the offence.

Revenue NSW said that because of the "technical" breach it would withdraw "fail to comply" penalties believed to be worth tens of millions of dollars.

"Fail to comply" fines were issued for a range of alleged offences from carpooling to public gathering.

They varied in severity but included fines of between $1,000 and $3,000 (US$670 and US$2,000) a piece.

Those who have already paid the fines will be reimbursed, Revenue NSW said.

Redfern Legal Centre, the legal aid organisation that brought the court challenge, hailed the ruling as "momentous".

"This case is about more than just two people's fines. It is about the need to properly adhere to the rule of law, even during a pandemic," centre solicitor Samantha Lee said before the ruling.

The group alleged that the fines were also disproportionately issued to people living in poorer neighbourhoods and regions.