Greens leader Shane Rattenbury, announcing the housing policy at the Polish Club in Turner on Sunday, said it would be the territory's biggest ever investment in affordable housing.

He said the temptation after the stress and uncertainty of recent months was for people to want life to return to normal, but the "old normal" wasn't good enough.

"We must instead seize this moment, when everything around us is changing, to build a better normal," he said.

The Greens want $200 million spent on funding 600 new affordable rental properties for community housing, and another $200 million spent on 400 new public housing properties.

Mr Rattenbury has been a minister in the Labor-Green government in Canberra for eight years, including two years as housing minister from 2012.

But he said it was the nature of shared government that neither party got everything it wanted. The Greens had pushed for more social housing, he said.

"This is a point where we as the Greens get to state what our priority is as opposed to, I guess, one of the negotiated outcomes that come out of the two-party arrangements," he said.

"What we're setting out here is [that] one of the most important things that needs to happen in the four years is to make this investment in housing."

The Greens policy also calls for more than $5 million a year extra for homelessness services, a funding boost of 20 per cent, on top of $9 million for new beds and specialist mental health and disability workers in homelessness services.

And they want $8.5 million spent on the long-planned supported-housing facility in Curtin for people with mental health issues. The Uniting Church has offered land for the MyHome project, which was in the Greens' coalition agreement with Labor after the last election.

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Also in the Greens' homelessness policy is $8 million for another 20 social housing homes at Common Ground in Gungahlin, and $300,000 to commission initial work on an Aboriginal-controlled community housing organisation.

The ACT's budget is deep in the red, with big projects already promised, including light rail to Woden, a new hospital and a new theatre centre. Mr Rattenbury said those projects were already committed and a $400 million housing spend would not elbow them further back in the queue, but it would come before a new stadium or convention centre.

"There is capital money available each year in the budget that's unallocated," he said. "Budgets are all about priorities and we think this is a central priority."

He also pointed to the cheap cost of borrowing and said the housing spend would create jobs at a time they were needed.

Social and affordable housing was a very good use of money, unlike the federal government's "McMansion expansion" home renovation grants, he said.

Mr Rattenbury's offsider, Caroline Le Couteur, will retire from the ACT Assembly this year, leaving the Greens with just one sitting member and battling the Liberals over the Molonglo-based seat being vacated by Ms Le Couteur. The Greens have put up community worker Emma Davidson for the spot.

The Liberals will be hoping the strength of Giulia Jones and Jeremy Hanson in the seat helps them snatch the seat from the Greens, a must-do if they are to have a chance at taking government.