A combined Australian-New Zealand navy would need more coverage

What would the navy of a united Australia and New Zealand look like?

It would broadly combine both fleets and grow somewhat larger because the merged state would have an even wider maritime area to cover.

The answer treats geography as the decisive force-structure driver. A combined navy would initially resemble the two real fleets added together, but political union would not make the surrounding ocean smaller. In the early twentieth century that pressure would favor a respectable cruiser force able to patrol widely. The modern equivalent is more major surface combatants than a simple two-fleet total might suggest, because coverage and availability must be maintained across the merged state's larger maritime area.