Prostantheras (native Mint) seem to need a crazy amount of water to survive

I have a bunch of prostanthera rotundifolia and about half of them dried up and died very suddenly after summer ended. I probably should have watered more often in summer but I am finding even now that it’s cooler, I am having to water regularly and a lot (even when it’s raining, unless it’s very heavy rain).

One of them was still drying up and only recovered after the recent deluge of rain yesterday and last night.

Is there something wrong with the soil? Should I add a soil wetter?

Location is full sun in summer and part shade in winter. In South West Sydney so very heavy clay soil.