To some, the Sirius building in The Rocks is a rare example of architectural brutalism, but to others it is simply an eyesore sitting in one of Sydney's most scenic suburbs.

But a NSW government decision not to heritage list the building has been labelled as "extraordinarily disappointing" by the president of the Institute of Architects, which says it's historical and social significance has been ignored.

The Sirius public housing building will not be heritage listed. Credit:Wolter Peeters

NSW Environment and Heritage Minister Mark Speakman announced on Sunday the building would not receive heritage listing, despite recommendations by the Heritage Council.

The council has been lobbying the government to list the building as heritage protected since February, due to its architectural and cultural significance.