In Australia is it not possible to install a US Water heater?
Apart from the question of whether it's got the appropriate certifications, etc. to be installed in Australia, which is a legal rather than practical matter...

It's a 240V-only appliance, it should be just fine. It is normal for everything operating on the US supply at 120V from ground to be rated for 250V to ground, so the insulation (electrical) should be safe with a 240V single-sided supply rather than a 120-0-120 supply. But your local electrical and/or plumbing inspector may object if it lacks the correct documentation for the local area.