New South Wales Premier Nathan Rees has told a gathering of delegates from more than 80 cities that sitting in traffic in Sydney is like being in love.

Mr Rees has told the opening of Sydney's Metropolis Congress that the city's transport systems are "stressed" but the congestion problems could be worse.

"Congestion is a concern for all Sydneysiders," he said.

"If you think you are in traffic, you are in traffic. It's not a relative concept. It's no good for me saying, 'Oh, it's much worse in New York or Paris'.

"It's like being in love. If you think you are in love, you are in love. If you think you are in traffic, you are in traffic.

"So for those of you who are from overseas, and those of you who are from other major cities who have jumped in a taxi this morning or been on a bus, please get into the ear of some of our journalists and let them know that the congestion in Sydney is not as bad as it could perhaps be."

'Water secret'

Mr Rees also told the audience his predecessor, Morris Iemma, had told him not to reveal the extent of Sydney's water crisis when he was water minister.

The new Premier said Sydneysiders would be "drinking mud" if not for the city's water restrictions.

"The former premier wouldn't let me say this, but I can say it now: we transfer roughly half of our water supply each day up from the Shoalhaven River in the south," he said.

"In February last year, in the middle of the worst drought in 100 years, if we hadn't been transferring water from that river and if we hadn't had water restrictions on, our water supply would have been down to 7 per cent."

"That's scary. That means people [would be] drinking mud."

Mr Rees said the city's planned desalination plant was needed to guarantee its water supply independent of rain.

Train wreck

The Premier also promoted the state's biggest infrastructure project, the Epping-to-Chatswood train line, at the conference, despite today's revelations that its cost has blown out again.

Tests have found the underground line will be too noisy for commuters. Media reports suggest noise levels in the trains on the line reach 90 decibels, which is as loud as a 737 jet coming in to land.

RailCorp is refusing to release the results of the tests. It says its plan to fix the problem will cost extra.

The line was originally due to run from Parramatta to Chatswood and open in 2006 at a cost of $1.4 billion, but it was cut in half in 2003 and costs have since soared to $2.3 billion.

The State Government says the opening of the line will not be delayed any further.

The full service will not open until mid-next year but shuttle services are due to begin in February.

The rail line is the state's biggest infrastructure project.

The Metropolis 2008 congress was opened by actress Cate Blanchett, who is the joint artistic director of Sydney Theatre Company.

Mayors, governors, senior city officials, academics and industry leaders are attending the four-day event, which is due to focus on issues affecting cities, including climate change, infrastructure, leadership and urban renewal.