What I don't fully understand is how she ever managed to agree to the decision to move to the other side of the world when she is so very close to her family in the UK. It really isn't just a 'Oh let's see how it goes for a year' . If you are so attached and close to your family. It was never going to work.
Because she's got a manipulative husband, the type where everything is great just so long as he's getting his own way (hence this latest flash of anger when she raises the prospect that he won't). These types are basically salesmen, adept at selling a dream and future-faking. There was never any intention on his part of returning home after a year. That was just and empty promise made as part of "securing the sale". The first part of his plan was getting her there, the second part will be trying to make her stay, including dragging it out "a bit longer" over and over, until the life back in UK has been lost because people have moved on and the DC are settled in Australia. At which point he'll refuse to let the DC leave and simultaneously will have also been grinding OP down about UK being shit/her missing DC if she goes, her being "selfish" etc. So she'll end up depressed and thinking that, unhappy as she is, what's the point in going home because it won't be any better there.
Part of selling the Australian dream would have been him going on and on endlessly about what a fantastic opportunity it is, how XYZ will definitely be so much better, how nothing can possibly go wrong and about how shit the UK is, how it's going downhill etc. All her worries will have been dismissed and ridiculous promises made (you can come back to UK every year and your family can all come here. A bit like his "go home and I'll visit lots" when she voiced her unhappiness, as if it's that simple and that fixes everything that's wrong). There'll have been some passive aggressive digs about her not being adventurous, being "too close" to her parents and clipping his wings by "her refusal to experience growth". Combine that wheedling, minor sulking and constantly going on and on whenever she's wary, with major excitement about Australia, constantly bigging it up, mega happiness when she mentions about ok maybe we could go, as if he's just won the lottery, and it's not so hard to get manipulated into decisions without fully thinking them through. Her marriage will be happier when he gets his own way and low-key not-so-great when he doesn't. So she convinced herself it's her decision too to move to Australia (because she doesn't see the manipulation), that it's what she wants. Dismissing her own worries like he dismissed them, because that's what makes her marriage run smoother and because he's made her start to doubt herself, feel silly for her concerns, question her beliefs and her ability to "choose correctly".
Men like this go through life this way, constantly. Whether it's something huge like a move to Australia or something more minor like whether to go out/stay home this Saturday, that ultimately benefits him primarily whilst she compromises as always, telling herself it doesn't really matter. It's a pattern that's played over and over in small ways throughout the relationship, essentially training the spouse to accept it. This time though, it's a huge thing, she's lost one of the most important things to her (her extended family) and realised she's made a huge mistake going along with his plans.