Sydney's iconic Sirius building has been sold to developers for $150 million, the NSW Government has announced.

Key points: The sale comes amid a strong public campaign to save the building from developers

The sale comes amid a strong public campaign to save the building from developers The proposed development will include 89 apartments and commercial space

The proposed development will include 89 apartments and commercial space The Government says the sale will help building social housing for 630 people

The brutalist landmark in The Rocks had been used for social housing since it opened in 1981.

However, the last resident was moved out last year.

NSW Housing Minister Melinda Pavey said the building had been sold to a company called Sirius Developments Pty Ltd, which was owned by investment firm JDH Capital.

She said the proposed "refurbishment" would revitalise the existing building.

"This is a great outcome that will see $150 million injected directly into building new social housing dwellings," Mrs Pavey said.

"This is expected to provide housing for around 630 people, helping the most vulnerable members of our community."

The view from inside an apartment in the Sirius building. ( ABC News: Riley Stuart )

The building's sale comes amid significant public campaign to save it from developers.

Despite the advice of the Heritage Council of New South Wales, the State Government decided against heritage listing the building in 2017.

That sparked a challenge in the Land and Environment Court, which was ultimately unsuccessful.

The building was sold following a tender process that saw "significant national and international interest", the Government said.

It said the proposed refurbishment would deliver 89 apartments — 10 more than the building's current 79-apartment configuration — plus retail and commercial spaces.

Mrs Pavey said the sale of 189 dwellings in nearby Millers Point had netted more than $750 million.

This money had gone towards constructing 1,500 residential units, of which 1,300 had already been built.

Developer paid 'a handsome price'

Chairperson of the Save Our Sirius Foundation, Shaun Carter, said he was "not getting too excited" by the sale, which he described as "secret squirrel, done behind closed doors".

Mr Carter questioned why the developer had paid a premium for the building, when others had only been willing to pay around $100 million to $120 million.

"Someone's paying a handsome price for this site," he said.

"The cynic in me says, what else are they expecting?"

Mr Carter used the Barangaroo development as an example of development in Sydney exceeding initial plans.

"If we look at this government and their actions with Barangaroo, that started as a project that was less than half the size that it is today," he said.

"If we use that as a measure, then we could be seeing Sirius demolished and towers on the site."

He said the Government had refused to engage the Save Our Sirius group, which still hoped the building's brutalist character would remain and be heritage listed.

Sydney Lord Mayor Clover Moore called on the Government to use the the proceeds from the sale to increase social and affordable housing in its redevelopment of the Waterloo Estate.

"While I'm pleased that this important brutalist building will be retained rather than demolished, I'm extremely disappointed that a public asset that was purpose-built to house people on low incomes has been sold for market housing after its tenants were evicted," she said.

The redevelopment of the Waterloo public housing estate contains plans to demolish more than 2,000 to build 6,800 dwellings, of which 5 per cent will be allocated for affordable housing and 30 per cent for social housing.

Ms Moore said the City of Sydney's alternative proposal for the Waterloo site included a "more humane housing mix, with 50 per cent social housing, 20 per cent affordable housing and just 30 per cent market housing".