Australia - New Zealand Open Skies
This should be implemented in-game. From my research, any Australia-based airline can operate domestic New Zealand routes and vice-versa. Additionally, fifth freedom routes are easier to obtain under the greement. So here's an example:
OASYS, based in SYD, would be allowed to base planes in AKL and operate an AKL-CHC route.
Air New Zealand, based in AKL, would be allowed to base planes in SYD and operate SYD-MEL, for example.