Sydney will end up a mess if planning action is not taken now, NSW planning minister Pru Goward says.
Figures from the Department of Planning and Environment predict an extra 1.6 million people are expected to move to Sydney by 2031, taking the city's population to six million.
The report released on Thursday predicted another two million people were expected to call NSW home in the next two decades, when the state's population would reach 9.2 million.
Ms Goward said the figures send a clear message.
"If we don't plan for this crisis, and particularly plan for growth in one- and two-person households, we're going to end up with a city in a mess," she told Macquarie Radio on Thursday.
"We have to take these population projections seriously."
She said the projections were a wake-up call for planning, for communities and for local government.
Peter Phibbs, Professor of Urban Planning at the University of Sydney says the city is going to see a lot more older people over the next 20 years.
"There's going to be almost a million people 65 years or over in Sydney by 2031."
He said the biggest challenge for planners is keeping Sydney an affordable city - especially for younger people.
The one-person household is the fastest growing household - especially in the CBD - growing at annual growth rate of two per cent, he said.
"A lot of younger people might be looking at smaller apartments in other cities," he told ABC radio on Thursday.
Planners needed to "think hard" about designing an age-friendly city, he said, as well as the types of housing available.