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According to this article King Charles IV of Spain in the 1790's planned to invade Australia and invade the British colony in Sydney Cove with a fleet of 100 ships.
If this invasion had occurred, very likely the British would have tried to recover Australia from the Spanish. It would depend on wither the re-conquest would have been worth the effort, given the Napoleonic Wars. If the Spanish managed to keep Australia, they would have definitely colonized New Zealand as well.
Personally is see a Spanish Australia being a antipodean version of Argentina, which would have declared independence during the 1890's.
This idea would make an interesting Timeline, if anybody wants to go ahead with creating such a timeline. I am happy to help you with it.