That's pretty much almost what the Labor Party proposal does. De-facto is almost identical in legal benefits to a registered marriage (there are a couple of edge cases, but I'm not sure what they are) already. The only difference is that you just have to fulfil the conditions for de-facto, instead of registration. And you don't actually need to even tell the state about fulfilling those conditions - it can be done by presenting evidence later down the track if it matters.

I assume that conditions for becoming de facto are defined in the Family Law act, but it may well be Common Law, or some combination between - I don't actually know.

You are currently allowed to hold whatever ceremony you like, you're just not allowed to call it a "marriage ceremony as per the Marriage Act".

That's why I say, it's a fairly good semantic dodge around the bullets. They can't abolish Marriage, it'd blow up. They can't let gays Marry, it'd blow up. But they can let gays have all the benefits and protections of marriage, by applying the de facto situation to gays.

Given the ratio of de facto marriages vs marriage ceremony marriages is almost certainly continuously increasing... That basically gives the situation that you describe, especially if the last edge case differences between de facto and ceremonial marriage are removed.