Clover Moore is appealing to Barangaroo developer Lendlease to buy and preserve the Sirius building for social housing.

But the lord mayor's appeal may fall on deaf ears, with Lendlease insisting the building is a matter for the state government, while the government says it wants to start selling the building this year.

Cr Moore wrote to Lendlease chief executive Steve McCann this month requesting a meeting to discuss using the Sirius building to meet Lendlease's affordable housing obligations in the inner city.

As part of its planning approval for the Barangaroo South development, Lendlease is required to provide 39 affordable housing units at Barangaroo, and another 25 affordable units within five kilometres of Barangaroo.