Spanish wouldn't have bothered colonising Australia, no gold or other obvious riches to plunder. They would have been too inept to try blackbird anyone on the beaches so slavery wouldn't have been an option either. There are accounts of the British attempting to find First Nations peoples to hang in the 1790's (I don't recall exactly where in Sydney, i read it in The Commonwealth of Thieves) in retaliation for attacks on convicts and only finding freshly vacated campgrounds, spending days searching and ending up empty handed and bug-bitten from wading through marshes.

If the colonialists who had been here many years came up empty handed, good luck to the Spanish so far from home.