New apartments flooding the market in Sydney are actually driving up rent in some areas, according to the latest report from the New South Wales Tenants' Union.

Key points: Price for new leases rose about four per cent in the year to September

Price for new leases rose about four per cent in the year to September 5,200 units have been built in the Sydney, Parramatta and Rockdale areas in the past year

5,200 units have been built in the Sydney, Parramatta and Rockdale areas in the past year NSW Opposition says taxation, incentives also affecting prices

The union's Rent Tracker report, which analyses rental bond data, shows the price for new leases rose about 4 per cent across Sydney in the year to September.

The union's research and advocacy officer Leo Patterson Ross said rents were growing faster for apartments than houses, even in areas where thousands of new units were being built.

"In places like Rockdale and the CBD and Lane Cove where lots of apartment are actually being built we're still seeing the price rise for new leases ," Mr Patterson Ross said.

"That's because these are new dwellings, so they're replacing older, less pricey stock and that brings the average and the median prices up."

While the report has not drilled down into specific suburbs, it states that there were 5,200 units built in the Sydney, Parramatta and Rockdale local government areas in the past year, and new rents went up five per cent in each of those areas.

In the July to September quarter, the median unit rent lodged with the bond board was $525 a week, eclipsing the median $520 a week rent lodged for housing leases.

Shadow treasurer Ryan Park said the report showed those trying to save a deposit to buy a home would not find it easy in the rental market.

"What it tells us is even if you are struggling to get into the home-ownership sector, you are doing it just as tough in the home-rental area and that makes it very, very difficult," he said.

Supply not only factor: Opposition

Mr Park said the report was more evidence the NSW Government's single-minded focus on boosting supply would not be enough to take the pressure off tenants.

"I don't know what more evidence the Government needs," he said.

"We've been saying for a long time supply is part of the equation but so is taxation, so are incentives."

But, while increasing rent may hit the hip pockets of those struggling to save a deposit or pay the bills, the tenants' union said the rate of increase had actually cooled off.

"Over the last 20 years the average increase in new rents has been about 8 per cent in Sydney, so 4 per cent is actually a lot softer," Mr Patterson Ross said.

NSW Premier Gladys Berejiklian has vowed to make improving housing affordability one of her key priorities.

The ABC has sought a response to the report from the new Planning and Housing Minister Anthony Roberts.