We're an Australian Family travelling to Maui or the Big Island soon. Will be be able to escape from the drought and water rationing there or are they suffering as well? Also, we are not enthusiastic hikers so is it easy to get around by car?
Oahu, and to a lesser degree Kona, have some water issues. I don't think they are near the magnitude of the drought that has been in AUS, rather something that is being addressed by conservation efforts. Go to Hilo if you want water.
There are plenty of roads. You won't have to hike anywhere if you don't want to.

Stay in east Maui or Hilo, Hawaii if you want nearly daily rain. (Guaranteed for Hilo). No water rationing in Hawaii anywhere. You will need a rental car to get around Maui or Hawaii. No need to do any hiking if you don't want to.
Hilo has been experiencing a drought-but only by Hilo standards. I didn't rain for an entire week about two weeks ago.
You don't have to worry about water rationing, but using only what you need, especially in the drier areas is always helpful.
what do you want to do with the water?!? seriosuly...?
I live in Australia, in a city not the country, and the water restictions don't affect me so much that i would look forward to NOT having water restrictions on holidays... We can only water the garder a few days a week and if you want to re-fill your swimming pool its a complicated issue.... But we still shower daily and have plenty to drink!
Are you looking forward to watering some plants willie-nillie and washing your hire car?
Please explain!
I thought it was a commendable stab at dry Aussie humour, without a full understanding of this being an irony- and sarcasm-free zone; everything is taken literally, or in the case of Hawaii, littorally.

Given the intellectual abilities of so many Aussies met on the road it seemed like something they would ask.
While i agree with ian that the TT can be a rather "irony- and sarcasm-free zone"
and that things tend to be taken literally here because we lack the body english to tell if someone is trying to be funny or just an idiot....
BUT i also see no irony in that OP's question... sounds like they are sayd "we are going on a holiday and our main concern is if there is water restrictions and if we have to walk around"