A spokesman for JDH Capital said the company was aware the building"held a special place in the heart of so many Sydneysiders".
"We have assembled an all-Australian,first-rate team of architects,planners,heritage consultants and builders to ensure Sirius is more sensitive to the character of The Rocks while retaining the building's iconic features,"the spokesman said.
"Once complete,we believe that the revitalised Sirius will be a building that all of Sydney can be proud of."
The firm's concept design,which is yet to go through a formal development application process,centres on refurbishing the existing building to deliver 89 apartments,as well as retail and commercial spaces on Cumberland Street and on Gloucester Walk.
Save Our Sirius Foundation chairman Shaun Carter said the organisation was not yet convinced the building had been saved and would wait to see further plans.
"We think the devil is in the detail,"Mr Carter,an architect said."There is so much here we don't know."
City of Sydney mayor Clover Moore urged the government to invest the proceeds from the Sirius sale into building more social and affordable housing dwellings as part of the redevelopment of the Waterloo public housing 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,"Cr Moore said.
“We are facing a housing and homelessness crisis in the city,with only one per cent of homes in Sydney classified as affordable and the numbers of people sleeping rough on our streets increasing."
Ms Pavey said the government's sell-off scheme had been vindicated,with the proceeds netted from the Millers Point and Rocks sales used to build four new homes for every one sold.
"There are around 55,000 people on the social housing waiting list,"Mrs Pavey said.
"The sale of the 189 dwellings in neighbouring Millers Point since 2014 have delivered more than $750 million for more than 1500 new residential units,with around 1300 already constructed."
Among those to benefit from the sell-off is septuagenarian couple Hans and Gloria De Bruyn,who until 18 months ago were living with their granddaughter while waiting in the queue for social housing.
Mr De Bruyn,who is on a disability pension,said he and his wife were"very happy"to have secured a new social housing unit in Colyton,near Mount Druitt,which had been built using the scheme's proceeds.
"This place makes a big difference to our lives because we're close to everything and our family,"Mr De Bruyn said.
"It doesn't look like housing commission at all. We are very happy with it."