I live in Canada but I imagine the same would apply.
In the late autumn, when the onions are large the tops usually start to fall over. I then dig then and leave to cure for a couple of weeks before storing. That is, if they are storage variety, if not start eating.
If the tops don't do as above, I tread on them gently to bend top over and then when further along, do as above. You must let the necks dry before storing or they will rot.
I grow enough to last the winter and braid them or put in mesh bags in which one buys onions, and hang in cool room. I look for small neck onions and good keepers.
Happy eating. Clementine

Where you are you can expect to be lifting your onions in late August early September. You want them to have as big a bulb as possible. Some varieties of onion bend over at the neck others don't I certainly don't intentionally bend them over. Make sure you use up first any where the neck doesn't close up.

Hi Thank you I will leave them a bit longer, I was a bit worried that the long stem ment they were bolting.

No they always grow a long stem.