The State Government has launched extraordinary legal action against Mercedes-Benz and two other companies, arguing Perth's gas-powered buses were unsafe for passengers and people near them.

The Supreme Court writ, lodged by the Public Transport Authority this week after a series of fires on the buses, claims the OC 500 LE model buses failed to meet safety, design and longevity requirements.

The PTA alleges manufacturers Mercedes and Volgren and supplier Evobus were negligent and breached their duty of care in designing and building buses that were not "reasonably safe to those travelling upon or within the vicinity of those motor buses".

The writ, which has not yet been served, also claims Mercedes breached its contract and that Mercedes and Evobus engaged in "misleading and deceptive conduct".

There were three major fires on the OC500 LE buses in Perth between December and March, taking the total to 14 in five years.

No one was seriously hurt in the fires and the buses are still on the road after the PTA developed a series of engine-bay modifications, including an improved fire- suppression system.

The OC500 LE model, which is powered by compressed natural gas and operates only in Perth and Sydney, was added to the Transperth fleet in 2005. The buses now carry about 35 million passengers a year.

PTA managing director Mark Burgess said negotiations were continuing with Mercedes.

He said the writ was filed to safeguard against any possible expiry of limitation periods for claims.

A Mercedes spokesman said the matters were being investigated and it would be inappropriate to comment further.

Former transport minister Alannah MacTiernan said yesterday she had been "locked in" to using Mercedes-Benz as the supplier for the buses when Labor won government in 2001.

"The previous government had entered into a contract with Mercedes-Benz to supply diesel-powered buses," she said. "We were concerned about using diesel, particularly in the inner-city, and wanted to use gas instead.

"We were locked into the contract but Mercedes-Benz told us they could deliver high-quality gas-powered buses."