They would have to go through the recruitment process.
The only way they would continue to operate the 737 is on a temporary basis until current mainline crew were trained. The J C crew would be issued a seniority # at the bottom when they joined, and once a current mainline pilot was trained in seniority they would drop back to SO.
Can’t see it in the short term though. If they did elect to have a NZ base for mainline, which I also can’t see happening, it would be open to all mainline crew to bid for, meaning the JC crew could still end up as SO’s in Australia.
Hopefully for all th operators, the reduction in seats will allow prices to get where they should be!