The behaviour of travellers between Perth and Bunbury could hold the key to whether a proposed high-speed rail line is extended to the rest of WA's South West.

The long-term project — to which state and federal authorities have contributed $8 million — would result in fast rail connecting WA's capital with its second-largest regional city. 

A feasibility study released in 2010 put the cost of the project at close to $1.2 billion and estimated it would reduce the travel time between the two cities to around 90 minutes.

The WA government is now seeking a consultant to study travel behaviour and patterns, which could influence its potential extension to Busselton.

A map attached to the tender documents from the state government Public Transport Authority shows Busselton as a potential connection consideration for faster rail.

Welcome addition

Busselton Mayor Grant Henley said the return of a rail service would be welcomed by residents.

"There are a lot of people in the local community who commute to Bunbury on daily basis and there would be a large take-up of that, but also people heading up to Perth," he said.

Mr Henley said there was the opportunity to establish a train service outside the main centre of Busselton that could be met by a bus or tram service.

A government spokesman said while the current planning project was limited to linking Perth and Bunbury, it recognised providing a connection further south to Busselton could be sought in the future.

"Accordingly, the investigation will look to make it easier for such an extension to occur in the future if required," he said.

Does the business case stack up?

The study will underpin the project's overall business case, which will examine the social, economic, and environmental benefits of the project.

It is expected to be finished in 2024.

The research will also cover whether the proposed service will use existing rail infrastructure used by the Australind passenger train, or whether a new line will need to be built.

South West MP Steve Thomas said increasing public transport between Perth and Bunbury was a good idea but the business case had to add up.

"We haven't seen any sign yet that the previous business cases have changed, so I expected it [the faster rail service] to be a long way into the future," he said.

"The fact that the Australind is being upgraded, and it is a significant multiple-million-dollar investment from the government I think indicates the chances of moving to an alternative fast rail process is unlikely."

The current Australind service from Perth takes around 2.5 hours.