Although I do not know Australian law. I am sure something like this would be decided by each state separately. But what I would do, other than check stat government web sites, is just to see if bikes with cruise controls are sold that way in Australia. If the manufacturers do not remove them then they must be legal for the bike to be approved for sale.
I have one of the cheap throttle locks that tightly grips the grip and rests on the brake lever. I would not rely on it as a cruise control, but it is good for the two purposes I need: if I need to readjust my right mirror and to rest my hand briefly.
As I have arthritis I also have a simple palm rest, the sort that costs a couple of pounds on eBay etc., which does help. So maybe that is also something to consider. But sometimes I need to straighten my fingers and remove pressure, so momentarily let my hand sit flat on the grip and lever without having to apply any pressure to the palm rest.
Because I am covering the brake while doing that, I should be as quick, if not quicker, to respond in an emergency than if I were just holding the grip. I would also consider it more dangerous to ride on to somewhere safe to stop while in pain, than straightening my fingers for a couple seconds to alleviate it immediately. Circumstance permitting, of course.