This is just looking at water usage/freshwater available. Australia has fairly large rainforests for example. So a lot of water available. At the same time the population is pretty small for such a massive country. The main issue is that the water is in the “wrong” part of Australia. The UK on the other hand is densly populated. So usage is high, while the land although on average probably wetter then Australia is also much smaller.
You have similar things in other countries as well.
The UK has been inhabited for longer and thus many more of its aquifers may have been tapped.
Despite being fairly arid, Australia is just massive comparatively. In combination with number 1, it may have huge aquifers under uninhabited arid regions that are close to full.
The UK is more water stressed than Australia ? That seems…unlikely ? Counter intuitive anyway
This is just looking at water usage/freshwater available. Australia has fairly large rainforests for example. So a lot of water available. At the same time the population is pretty small for such a massive country. The main issue is that the water is in the “wrong” part of Australia. The UK on the other hand is densly populated. So usage is high, while the land although on average probably wetter then Australia is also much smaller.
You have similar things in other countries as well.
Two random guesses: