Have you seen this website? www.gardengrow.co.nz It gives an idea of what to plant out when. It says in Feb to start onions in seed trays for planting out to harvest July to September.
Onions take a long time - about six months - so if it were me, I'd plant them out now to harvest in autumn. If you leave it too late, I find they slow down so much over (the cold Canterbury) winter that they aren't ready until late spring, by which time they're all desperately going to seed.