Last year saw the smallest increase in rents in eight years, but property analysts warn they are set to rise again.

A report by Australian Property Monitors says nationally, rents grew by only 2 per cent in 2009, making it the weakest year for national rental growth since 2002.

That compares with an average 12 per cent in each of the previous two years.

But APM economist Matthew Bell says rental growth is likely to pick up again this year.

"In terms of the relatively flat rental market, it's just been the weak economy," he said.

"Renters have been a bit less sure about their job security so it's just led to them not being able to agree to rental increases.

"But going forward there's still very low vacancy rates, there's still strong population growth and there's simply not enough houses being built to house the rental demand that's going to be there."

He says things could get more difficult for alot of people.

"If we don't start building enough houses to house the increased population and in general the increased income we're going to get from an improved economy, then things will start getting tough for a lot of people, particularly those who have lost jobs or had to move to part time work," he said.

"They'll just be forced to move a bit further out and away from those areas of employment."

The asking prices for rental properties in Sydney have risen by less than 2.5 per cent over the past 12 months.

In the last quarter, rents for houses remained unchanged, while tenants renting units paid an average of 1.2 per cent more.