Two Japanese firms that until recently were the front-runners for a multibillion dollar contract to build Australia's new submarines have declined an invitation to attend a gathering of top Australian naval officials and politicians this week.

The no-show by Mitsubishi Heavy Industries and Kawasaki Heavy Industries at an event called Australia's Future Submarine Summit, held amid intensifying competition for the deal, exposes a potential weak link in Prime Minister Shinzo Abe's more muscular security agenda: Japan Inc.

While Abe wants Japanese firms to vie for overseas orders after he lifted a decades-old ban on arms exports last year, such companies are showing little appetite for doing business in foreign markets after being restricted to local sales for so long, Japanese defense officials and experts said.