"As an architect I'm probably a good lawyer," the Sydney silk Bruce McClintock, SC, told the Land and Environment Court on Thursday.

But Mr McClintock could not help himself. "In some ways it is a shame that the architectural style has been given the title 'brutalist'," said Mr McClintock, speaking about the Sirius building on the edge of the Rocks.

"It is an architecturally sensitive building intended to be friendly to its environment," Mr McClintock said.

But the architectural merit of the Sirius building, built in the late 1970s to house public housing tenants displaced from the Rocks, is not what could yet save the structure.

Instead, a community group fighting to preserve the Sirius must convince the court to quash a decision by the then Heritage Minister, Mark Speakman, to direct that the Sirius building not be listed on the State Heritage register.

Mr McClintock, for the Millers Point Community Association, argued that Mr Speakman erred in considering that if the building was listed on the heritage register it would cause undue financial hardship to the building's lessee, the NSW Land and Housing Corporation.

Whether or not a heritage listing would cause undue financial hardship is one of the considerations the Minister is required to make under the Heritage Act, after they receive a recommendation from the Heritage Council that a structure is worth listing and therefore preserving.

The Heritage Council unanimously recommended the listing of the Sirius Building in a decision made public in early 2016. But when Mr Speakman decided against listing the building in July, he said listing the structure would deny the state about $70 million it could spend on other social housing.

Mr McClintock said this was not what was intended by the Act. Undue financial hardship "doesn't mean diminution in sale price ??? it means actual hardship," he said.

But barrister Stephen Free, for Government Property NSW, said Mr McClintock's interpretation would require the Minister and the Heritage Council to consider the financial position of any party to be affected by a heritage listing. That would be a "wholly unpredictable and accidental" circumstance, Mr Free said.

Mr McClintock also argued Mr Speakman failed to make a determination about whether the Sirius merited listing on the Heritage Register before he decided against its listing.

The legal tussle over the Sirius, which the government intends to sell to raise housing funds, echoes the strongly held opinions that have long been held about the building.

For the community group fighting to save the structure, the building is a tangible link to the Green Bans of the 1970s, and the need to preserve spaces in the inner city for working class residents.

For others, the building is an obvious eyesore. State Treasurer Dominic Perrottet has written the Sirius is "about as sexy as the car park at my local supermarket".

The acting judge of the Land and Environment Court, Simon Molesworth, reserved his decision after Thursday's hearing.