WA Labor is hoping to ride an ambitious public transport agenda into office, committing to deliver its multi-billion dollar Metronet rail system if elected in 2017.

Opposition Leader Mark McGowan made the pledge at Labor's state conference in Perth, arguing the project could be staged and would be affordable.

"By setting the right priorities and realistic time frames, we can afford it," he told the conference.

"In fact, the real question should be: Can we afford not to build Metronet?"

The big-spending infrastructure proposal comes at a time when Labor has lashed the Barnett Government over spiralling expenditure and debt.

WA has lost its triple-A credit rating, the state budget deficit will rise to $2.7 billion next year, and state debt is due to hit $31 billion next year before peaking at $36 million in 2018.

But Mr McGowan insisted the project would be viable if he was elected Premier in March 2017.

"We can afford Metronet because it's a long-term plan," he said.

"I won't be the premier who cuts the ribbon when Metronet's finished, but I sure want to be the premier to starts it."

Metronet to be built in stages

The project would be completed as the government could afford it, according to Mr McGowan.

He said the priority stages of the project would include completing the Airport Line linking Forrestfield to the city, extending the Joondalup Line to Yanchep, extending the Armadale Line to Byford and creating a circle line through Perth's southern suburbs from Thornlie.

There is no projected cost for the revised Metronet proposal. The original project was estimated to have a total cost of $5.2 billion.

Mr McGowan insisted the project was essential for dealing with the growing congestion problems of an expanding city.

"We need to plan ahead. If we don't, we're betraying future generations," he said.

"The endless urban sprawl can't continue as it has ... our suburbs are seizing up."

McGowan takes aim at Liberals

Mr McGowan took a swipe at the Barnett Government's record on public transport, highlighting the broken promise to deliver Max Light Rail to the northern suburbs.

"Can there be any starker comparison between us and the Liberals? The Liberals who never delivered their promise to build a line to Ellenbrook. The Liberals who never delivered their promise to build Max Light Rail," he said.

He also dismissed the Premier's focus on the Perth CBD, characterising major projects as "monuments and vanity projects".

He questioned what was being done for the families living in the suburbs.

"My opponent talks about transforming the city. But is he really?" Mr McGowan said.

"Do you change a city for the better by ignoring the suburbs where most of us live? Do you improve a city by failing to act on the congestion that threatens to choke it?"

Mr McGowan said Labor would work with councils and communities on Metronet to determine the right routes, stations and locations, as well as the potential for light rail in the city.

McGowan 'dreaming': Dean Nalder

Transport Minister Dean Nalder was deeply sceptical of Labor's revived Metronet plan, and said a reported estimate of $2.5 billion for the range of rail services was unrealistic.

Dean Nalder says Labor's focus on rail as the answer to congestion is flawed. ( ABC News: Andrew O'Connor )

He said Mr McGowan was "dreaming" if he believed he could deliver the rail lines to Ellenbrook, Yanchep, Byford and from Thornie for that amount.

Mr Nalder also said Labor's focus on rail as the solution to congestion was flawed, saying 42 per cent of vehicles in the metropolitan area were commercial vehicles.

With Perth's population expected to double over the coming decades, he said rail alone would not solve congestion.

"If we have just over half of our vehicles on the road now being commercial vehicles, then we must have an integrated solution to transport requirements in Perth. It cannot just be about railway lines," Mr Nalder said.

"So how they're going to do this is something I'm yet to see.

"I'm quite cynical about their story - that they're going to solve all the public transport, or all the congestion issues, through rail lines."

Mr Nalder said a rail line to Ellenbrook would be needed in the long term, but not for up to 20 years.