Light rail is more convenient, more comfortable and quieter than buses, according to a comparison study done for the Barnett Government in 2013.
The Department of Transport study also found buses would be unable to meet Perth's future transport needs effectively.
It said each light-rail vehicle could carry up to 300 passengers and it would take three articulated buses to carry the same number.
The comparison also found light rail used less energy.
On Saturday, _The Weekend West _revealed Transport Minister Dean Nalder was looking to replace the proposed $2 billion MAX light-rail system from Mirrabooka to the city with high-capacity buses using dedicated bus lanes.
The MAX project was promised by the Barnett Government at the last State election but was shelved in December 2013.
At the time, the Government said no decision about whether to proceed with MAX would be made before the next election, due in March 2017.
Shadow transport minister Ken Travers said the Government's transport planning had become a joke.
"The Government promised a light-rail system," he said.
"For them to deliver anything other than a light-rail system is a broken promise."
But Mr Nalder said the Government had not committed to buses over rail, merely that it was being investigated.
He believed buses could deliver "exactly the same experience for the consumer".
"We've got to explore that fully and build a proper business case for it," he said yesterday.
The comparison document titled Why Light Rail for Perth? said an efficient means of moving large volumes of people was critical to meeting Perth's growing transport needs.
"As a result of this growth and congestion, many key public transport routes, like Alexander Drive, are experiencing capacity issues and increased travel times as the existing bus services are caught in congestion," it said.
"As this trend continues, current and forecast demands indicate these services will be unable to effectively meet future needs, despite buses running at high frequency."
Property Council executive director Joe Lenzo said the uncertainty about public transport, such as light rail versus buses, was causing serious delays for developments.
Federal Labor MP Alannah MacTiernan said State Government funding for Roe Highway Stage 8 should be redirected to the MAX project.