MOVING container handling from the Port of Fremantle to Kwinana 14 miles to the south by end of 2030s at cost of at least A$7.2 billion (US$4.5 billion), reports the Australian Broadcasting Corporation.

Increased container trade at Fremantle Port means it is moving closer to operating at full capacity.

The relocation should happen before Fremantle Port reaches capacity - a scenario which a new study says would put a significant brake on Western Australia's economy.

The Westport Taskforce, the government body charged with planning and building the new port, has released its business case for the enormous Kwinana project.

It calculated that if nothing was done and Fremantle Port was allowed to reach capacity, more trade destined for Western Australia would have to pass through the eastern states, costing WA about A$5 billion a year.

The WA government has now committed A$273 million for detailed project planning and approvals, after which capital works contracts can be awarded, with construction potentially starting within five years.

A new port at Kwinana connected to upgraded road and rail infrastructure has long been pushed by Labor, as a necessary alternative to former Liberal Premier Colin Barnett's controversial and now-abandoned Roe 8 highway extension and continued use of Fremantle port.

Premier Roger Cook said the new port would be in addition to the AUKUS nuclear submarine maintenance site, and the manufacturing and renewables precinct in Kwinana, which is his electorate.

"As a local, and perhaps without a hint of a conflict of interest, I've always said Kwinana is the centre of the universe," Mr Cook told a business breakfast.

Westport's business case estimated the government would need to spend A$7.2 billion to deliver the Kwinana container port, although the final, likely higher, budget would become clear after all contracts were signed.