Is it just for career that you want to move to Sydney, or are there other things or friends, family etc?
DH and I moved to Melbourne whilst I was pregnant with DS. We sold our house in qld and got DS name down at a school when he was born. Over the next 12 months I became depressed, the living costs were so much higher than I could have imagined, even our increase in income didn't cover that and to buy another house we had to look so far out of town, in suburbs we didn't like for a ridiculous amount of money. We were constantly chasing our tails. I also realised it wasn't where I wanted DS to grow up - there was no way I would have known if I didn't try though.
We lasted just under 2 years when luckily DH managed a transfer back to qld with his company - a small drop in salary - but we didn't regret it for a second. It is so much easier financially up here and we are much much happier, even if our jobs aren't ideal.
That said, you have to be happy with your decision what is right for you, if you're having doubts it's probably for good reason and the moment things get tough you will prob find yourself wondering why didn't you listen to your gut etc..,
Not saying you shouldn't go - is there any way you can go and 'try' give it 12 months but give yourself the option to move back of it isn't working?