The whole story and situation seems really weird.
It's no secret that providing online poker services to Australians has been an offence under Australian law since 2001. I don't recall reading any provisions in the Act that prevent Australians from
playing online poker though and to the best of my knowledge Australian authorities haven't shown any real interest in pursuing any of the sites in the 10 years since the act was passed, likely because they're almost all based overseas outside of Australian jurisdiction. There's no ambiguity like there is in the US by the way - as the link above mentions, online poker is specifically named as a prohibited service.
The hypocrisy of the situation (whereby online sports betting is 100% legal, government grants monopolies to land based
casinos and political parties recieve huge income from the
gambling lobby) is something that's being discussed in
this thread BTW.
What I didn't realise was that the Merge network was based in Australia (I don't really play anywhere other than Full Tilt). If they're Australian based then it makes perfect sense for them to block Australian players - after all, they're in clear violation of the law
and within reach of the Australian authorities. The weird bit I can't work out is why the chose to open the site to Australian players in the first place, and why they chose last year to do it? They knew it was illegal for the better part of 10 years before they made that decision...