Perth's Max Light Rail project could be put off for decades if the WA Government proceeds with an alternative plan to use rapid transit buses, Transport Minister Dean Nalder says.

The Max Light Rail was an election pledge the Liberal Party took to the 2013 state election and would have seen Perth's CBD linked by trams to the northern suburbs, QEII medical centre and Curtin University.

But today, Mr Nalder said that system may be deferred for more than 15 years in favour of a bus network using express lanes.

"We believe for the next 15 to 20 years, as a minimum, we can provide the solution with a bus," he said.

"That's what the preliminary analysis suggests, and we actually now want to do all the full work to validate that."

Mr Nalder has defended the dramatic policy shift against widespread criticism, and has maintained the Max Light Rail project without light rail is not a broken promise.

When the project was first announced in September 2012, then transport minister Troy Buswell characterised it as "transformational", a project that would "be an important part of this city's transformation during a period of strong economic and population growth".

Originally promised to be operational by 2018, that start date was later deferred to 2022.

The project was initially estimated to cost $1.8 billion, but that later blew out to $2.5 billion.

The cost of rapid transit buses is roughly half that of light rail, with an estimated cost of $1.2 billion.

Mr Nalder insisted it was the same project.

"Everything that we're looking at is to see if we can't deliver exactly the same experience for the community with a bus as with the light rail," he said.

Bus network sub-par: Opposition

WA Labor has branded the change a broken promise.

Opposition Transport spokesman Ken Travers arrived at a media conference wearing a bus conductor's hat today, behind the wheel of a small car emblazoned with makeshift livery "MAX Light Rail".

He said the stunt was to illustrate how ridiculous the Government's transport policy had become.

"The Government promised a light rail system," he said.

"For them to delivery anything other than a light rail system is a broken promise."

"What we've got to understand is that the Government has clearly now set about a path to deliver a sub-par public transport system for the people of Perth, despite the promises they made at the last election."

Mr Travers said the Government's problems were not confined to Max Light Rail.

Four years after releasing its draft public transport master plan, he said the Government had achieved nothing.

"In those four years, the only thing they've done is to junk every one of their priority projects," he said.