It makes totally sense to keep them rear facing for as long as possible (why on earth did I think 6 months....)
It depends on the car seat too - some only rearface until 8kgs, whereas others (even some of the newer style capsules) are up to 12kgs.

I was told that it doesn't matter how long they're legs are as long as they're head isn't over the top of the seat, so DD would have still fitted into DS's capsule at 22months.

Also the countries that rearface up to 4years or so have different car seats and different ways of anchoring them to the car, so I probably wouldn't do that with the Australian car seats.