Stand by subs pledge, union tells Libs

South Australian shipbuilders have descended on Canberra to lobby Liberal MPs to stand by a pledge to manufacture submarines in Adelaide.

The workers are pushing for local builder ASC to be offered the contract to manufacture Australia's next generation of submarines, as confusion continues over a "competitive evaluation process" for the project.

Prime Minister Tony Abbott promised SA colleagues the shipbuilder would be able to put their hand up for the job, in the lead-up to the Liberal leadership vote on Monday.

But there are now doubts about how open the evaluation process will be to the government-owned shipbuilder.

Former defence minister David Johnston had promised the subs would be built in SA, and the Australian Manufacturing Workers Union has called on Liberal MPs to stand behind the pledge.

A lot of taxpayer money had gone into building an industry for the future of Australia's sovereign capability, assistant national secretary Glenn Thompson said.

"It is absolutely ludicrous that a tender ... not take place, and (submarines) not be built in Australia," he told reporters in Canberra on Wednesday.