Australia's next submarine will be much bigger than the existing Collins boats but the likely cost won't be known for years, a senior defence official says.
David Gould, general manager for submarines in the Defence Materiel Organisation (DMO), said he couldn't estimate the cost of a vessel not yet designed and could not endorse estimates of $36-40 billion in some studies.
Under the 2009 and 2013 Defence White Papers, Australia is looking to replace the six Collins submarines with 12 new vessels of greater range, longer endurance and expanded capabilities.
This is set to be Australia's largest ever defence procurement.
Mr Gould said the new boats would have to be larger than the 3500-tonnes Collins.
"It will be in my opinion larger, much larger. That's the work we are doing at the moment," he told an Australian Strategic Policy Institute (ASPI) national security lunch.
The 2013 White Paper reduced the acquisition options from four to two, ruling out the cheapest options of an existing overseas design or an existing model with some Australia-specific systems.
That leaves either an evolution of the Collins or an all-new design.
Mr Gould said Australia had reached agreement with the government of Sweden on intellectual property rights to allow a start on concept designs for an evolved Collins.
That work would be undertaken by German shipbuilder TKMS, now the parent of Swedish firm Kockums, the original Collins designer.
Mr Gould said TKMS would have to correct known defects with Collins and propose improvements.
"But we will not allow them to increase the diameter of the pressure hull - to do so would clearly cross the threshold of a new design," he said.
Mr Gould said two concept designs should have been completed in two years and a decision made on the propulsion system for the first two or three boats.
There would be a much better handle on costs of design and construction.
"But we will not have contract quality costs for a build by any stretch of the imagination at that point," he said.