Hiainu means: Thirsty.
I’m going to run out of things I know to say about ‘hia-‘ and statives!
The Te Aka entry for hiainu gives the example sentence:
This is an example of a sentence with a conditional clause using the verbal particle ‘ki te’.
‘Whakainumia’ is the passive form of the verb ‘whakainu’ - ‘to give drink to’. The passive form is used here because it’s more common to give commands with transitive verbs in the passive form in te reo Māori. The passive form of a verb adds the passive suffix (called the whiore - or tail). There are 17 different options for the passive suffix and each verb can only use a subset of them (often 1-3 are valid across different dialects). In general, you’ll need to use a dictionary to identify the correct way to construct the passive form of a verb (although I’m given to understand that some dialects may apply more general rules, but if that’s the case I haven’t learned them yet).