Transport Minster Dean Nalder has asked his department to identify the best site for a second major airport for Perth.

He said the city must start planning for how it would work when its population reached over four million and believed identifying the correct spot for a new airport was a key part of the task.

The timeline for a new airport could be 30 years or more into the future but Mr Nalder said it was crucial that suitable land was identified early given the complex considerations around flight paths and major road and rail connections.

He believes a second airport - which could compete for cargo and passengers with Perth Airport - would not be needed until well after Perth Airport completes its third runway project, which many analysts believe will future-proof the city for decades.

"I was living in Sydney when they went through the debates about Badgerys Creek 20 years ago," Mr Nalder said. "They were still debating it this year.

"To me, you don't want to be facing that situation where you're having a crisis to force an outcome.

"I've asked the department to consider where a second airport could be built."

He said it was too early to speculate on specific locations.

Pearce RAAF base in Bullsbrook has been floated as a possible alternative to Perth Airport, with airline executives reportedly keen to explore it as an option to take pressure of busy mid-week landing and take-off slots.

It is believed the Department of Defence is cool on the idea.

A better location may be in the southern suburbs where there are big areas of vacant land close to major roads including the Kwinana Freeway.

"Some people are saying Perth will have 3.5 to four million by 2050 but there are other groups whose modelling suggests up to five million people is realistic," Mr Nalder said.

"That's a massive change from where we are today and I think we need to understand that, which means the departments I'm resp-onsible for need to work closely with the department and Minister for Planning so we're in sync.

"I think that a second airport would only get legs to the extent that the current airport can't cope.

"I know that a third runway will provide increased capacity and take that out for a number of years. That's fantastic and we support that.

"If we talk about a city with a population of five million, we're talking about a city the size of Sydney.

"It becomes part of a greater plan around our freight corridors, our ports and getting all that commercial traffic moving freely and as productively as possible."