I spent 6 days in Australia, and applied the Sun block in the trip (30+ is quite the max I can find in any phamacy shop), but I still get serious sun-tanned.

Better to prepare 60+ or even 100+ in HK before going there.

I went to Tasmania for brush walking.

It is not that common to buy any sun block more than 30+ locally. So better to buy stronger sun block in HK.