Defence Minister David Johnston has indicated the next fleet of submarines will not be designed in Australia.

The Government promised before the federal election to build 12 new submarines in Adelaide, but there has been speculation it will instead opt for a Japanese-built model.

Senator Johnston said the Government needed to make a decision before the release of the Defence White Paper next year.

"We are under such time constraints to avoid a capability gap and I cannot afford to not have submarines in the water," he told Sky News.

"This country cannot afford that."

Senator Johnston said the the previous Labor government had no design plans in place when it lost last year's election.

"We've wasted six years. We have to come up with a solution that solves the time-schedule problem," he said.

"When I got into Government, when I said, 'tell me about [the] SEA-1000 [submarine building program], tell me how far along the track we are,' [I was told] 'we're not far at all – there's not a contract, there's not an obligation, there's not a design'."

In his first bilateral meeting with Japanese defence minister Akinori Eto in Tokyo earlier this month, Senator Johnston formally requested Japanese cooperation in a deal.

The Japanese minister said they would first talk about how a deal could benefit both countries and proceed from there.

A number of senators, including some from the Coalition, have urged the Government against purchasing overseas submarines.