True - it's not legislated but it's been a big issue in the past for CAMRA and they've successfully kept the pressure on pubs to keep half-pints half the price of full-pints
That's how I remember it as well, all pubs sell half pints for half the price of a pint, which is great as you can just order a half if you feel like, without worrying what's most economical, and then order another half if you decide to stay.
Whereas in Germany most places sell 'a small one' (ein kleines, 300ml) or 'a large one' (ein grosses, 500ml), some places even sell 400ml but don't call it 'a medium one' but just '04' instead.
Obviously there are regional variations, in Cologne and Duesseldorf the standard for a koelsch or alt is 200ml, and Around Munich you get the Steins.
Bottles are always 330ml or 500ml, everywhere, no variations.
Pricewise it's often, but not always, better to order a large one, and often it's cheaper to order beer than water. There is a law that is supposed to prevent this, but reality is still far from it.
As for Australia, I'd say stick to the pints, the price would stay the same if they get substituted with 500ml euro measurements anyway, might as well take the extra 65ml.