The NSW government wants to halve the number of people sleeping rough across the state within six years as part of a global pact.

Premier Gladys Berejiklian on Wednesday signed an agreement to halve the homeless population by 2025, alongside the City of Sydney and other service providers.

Sydney is the 10th city to join the Institute of Global Homelessness initiative.

Social Housing Minister Pru Goward acknowledged it was a "very, very ambitious" target, however the government would not be investing any extra money towards reaching the goal.

"It's not just about the money. It's your sense of purpose, it's your ambition, it's how you spend that money," Ms Goward told reporters.

"It can be done and it will be done with intent, with purpose and with that great ambition."

The NSW government has already committed $1 billion to homelessness over four years.

Ms Goward said there were between 300 and 400 rough sleepers in Sydney but did not have a figure on the state-wide number.

Treasury's data analytics centre has now been given that task.

"What we're not clear on is how many homeless people there are out in our regional centres," she said.

The government's assertive street outreach program has housed close to 300 rough sleepers since May 2017.

Almost 95 per cent of those have maintained tenancies for more than 12 months, according to the government.

Institute of Global Homelessness chair Dame Louise Casey said housing was critical.

"To ignore the need for housing is just not sustainable," she said.

Opposition leader Michael Daley said he was wary of the "lofty statements" made by the government.

"If you don't know what the starting point is and you don't put any money into it, that makes it a difficult announcement doesn't it," Mr Daley told reporters.

He said homelessness should not be a partisan issue nor part of an election campaign.

"We support any moves by any level of government to address homelessness; there's no money attached to it but there's an agreement to try and move forward," the Labor leader said.