We migrated with 6&3 year olds to Queensland. Cut off here is end of June, so my then 6 year old started year 2 in the uk and then restarted year 2 here in the January. Private school so there are some children 18m+ older than him - delaying a year or holding back is very common.
We arrived mid Dec which was ideal. The first 2 weeks is a jet lagged haze of hell sorting out rentals, mobiles, jobs etc. Then we had another 4 weeks of time to just enjoy being here as a family before school started, but that only works if one or both of you can be at home. If they’d have to go to vacation care anyway then they may as well go to school.
In terms of education, I’ve found it quite different. DS repeated a lot he’d done before in year 2 in terms of English and maths, but of course the history and geography is totally different. There’s also a big focus on non academic skills - resilience, confidence, communication and so on that has done the world of good for DS.
I will give you a word of warning though. I heard so many times before we came, “kids adapt so fast”. Well not always. School here were more honest and as they regularly take immigrants said it’ll take 6 months. That 6 months was pure unmitigated hell. DS behaviour (only at home) was diabolical - he was so lost but it made me question what on earth we’d done to him. He’s back to himself now, but I do wish someone had warned me this can happen. My 3 year old didn’t bat an eye, but older children don’t always settle easily - just my experience.
Melbourne is very expensive by the way - you’d need a pretty good salary!
Australia is great. The year before and after the move is very stressful, but the sun shining every day really helped! Good luck