Have another look at your tenancy agreement especially this section:
10. The tenant agrees to pay:
10.5 water usage charges, if the landlord has installed water efficiency measures referred to in clause 11 and the residential premises.
I believe it would also depend on whether the hot water is metered or just a % of the total. If it is metered and you only pay for what you use then it’s reasonable (subject to 10.5).