SA build 'could deliver low-cost OPVs'

Australia could build four offshore patrol vessels at "very marginal cost" if the commonwealth commits to construction in South Australia a year ahead of schedule, a defence lobby group says.

Adelaide-based ASC and West Australian builder Austral are the key competitors vying for the contract to build the boats, with construction set to begin in 2018.

But Australian Made Defence says an Adelaide-based build beginning in 2017 would cut costs by capitalising on ASC's existing Air Warfare Destroyer workforce as it winds down.

National spokesman Chris Burn cites the 2015 defence industries report, which says the four vessels could be added to the Royal Australian Navy project at "very marginal cost" compared with stop-start projects.