There has been “unprecedented” interest in buying the iconic Sirius public housing building at The Rocks on the Sydney Harbour.

Expressions of interest closed on Thursday afternoon as part of a New South Wales government plan to fund housing on cheaper land.

Stuart Cox from Savills, the marketing agency acting for the government, said there was “mixed” interest in keeping the building and knocking it down.

Interest from local and offshore developers was for residential apartments and hotels.

“There has been a significant level of interest and an unprecedented number of inspections,” Mr Cox told The New Daily on Thursday.

Shortlisted buyers will proceed to the public tender process, and the final sale is expected to be finalised by the end of the year.

Shaun Carter from the Save Our Sirius campaign said about 10 interested buyers had been in touch with him, which was “heartening”.

“The vast majority of those 10 were seeking to keep the building,” he told The New Daily.

Anybody hoping to knock it down would be “buying trouble” and have to get through picketers, he said, confirming the CFMEU green ban on demolition was still “100 per cent in place”.

The green ban means no unionised workforce can take part in its demolition, and was put in place after the government controversially denied the 1970s Brutalist building heritage status.

Mr Carter hoped the successful buyer would retain the building, seek to have it heritage listed, and provide housing for key workers in about 30 to 40 per cent of the building.

He was critical of the “secretive” EOI process and questioned why it wasn’t a public tender from the beginning.

A Property NSW spokesperson was critical of the green ban.

“A green ban has the potential to compromise the ability of the government to recycle proceeds from the sale to construct hundreds of new, modern and fit-for-purpose social housing dwellings across the state.”

The spokesperson confirmed the government received “strong interest from a diverse range of proponents during the EOI stage”.

Sirius has been vacant since February when the last remaining tenant, 91-year-old Myra Demetriou, was moved on.

Minister for Planning Anthony Roberts in December said it would have maximum height restrictions in line with the Sydney Harbour Bridge deck to protect harbour views.

“Any building proposed for the site would need to meet design excellence standards for its architectural, urban and landscape plans so that we can ensure we are creating a great place that fits well with the surrounding area,” he said at the time.

The building reaches 11 storeys and tapers down at the north and south.

Savills marketed the location as having “excellent demand for luxurious owner-occupier residential apartments”, and “significant demand for a new luxury hotel”.

“The Sirius Building represents a unique opportunity to acquire a rare freehold land parcel in the tightly held iconic precinct of The Rocks.”

The site would require “activated street frontage” along Cumberland Street – which could include commercial, retail or community facilities – under the draft regulations.

The maximum gross floor area would be restricted to 8420 square metres under the draft rules. If the existing building is retained, the maximum gross floor area of 7010 square metres will apply. The site itself is 3664.5 square metres.

It has a maximum limit of about 70 underground parking spaces.

Sirius currently has 28 studio and one-bedroom apartments, and 38 two-bedroom apartments. Eight of the apartments have three bedrooms and five have four bedrooms.

Its units and rooftop gardens have uninterrupted views of the harbour and CBD.

Sirius is part of the Millers Point sales, a program slated to deliver more than 1500 new homes.

It has so far resulted in the completion of 977 new units, while a further 255 remain under construction.

The waiting list for government housing is 10 years in parts of the state, with a shortfall of more than 55,000 properties.