Not 100% certain about the overwater requirements in Oz now for SET IFR, not there myself either and when I wasn't didn't give it a lot of thought as it wasn't something I was involved in. But i'm not sure it would be a major issue as there isn't a great deal of overwater to be done in Oz, but from what I remember you have to remain within gliding distance under CAR258 but the AIP allows you to conduct overwater outside gliding distance but no more than 25nm from a suitable landing area as long as all passengers wear a lifejacket (not sure if this means actually around their neck or having one in its pocket around a persons waist is permitted) and pretty sure life raft required as well.
And that is a good point about being able to carry less pax and/or cargo. I'm only really thinking about Domestic operations, but once again a good point to consider.