Opposition to the redevelopment of one of Sydney’s most controversial harbourside apartment blocks has ramped up as unions move to block work on the site.

The CFMEU construction union has placed an interim Green Ban on the Sirius building at The Rocks, meaning no unionised workforce is allowed to work on the building that overlooks the Harbour Bridge and Opera House.

The building, which has been used for social housing since it opened more than 30 years ago, has been slated for redevelopment into hundreds of private apartments by the NSW government.

“It is one of the last areas of public housing remaining in the district,” CFMEU NSW Secretary Brian Parker said.

“The top end of town moves in and ordinary working people are locked out of the suburbs they have called home for more than a century.”

The move comes as the Save Our Sirius campaign hit the $35,000 milestone in its crowd fund raising campaign to take the government to court over its decision to refuse heritage building status for the building.

The NSW government has rejected a heritage listing for the apartments – built in a Brutalist architectural style – because it would slash tens of millions of dollars from the site’s sale price.

“I am not listing it because whatever its heritage value, even at its highest that value is greatly outweighed by what would be a huge loss of extra funds from the sale of the site,” NSW Environment Minister Mark Speakman said in late July.

The sale funds would be used to build social housing elsewhere, he said.

Residents of the building and activists have planned to rally against the sale on Saturday.

AAP