Chrissy I would say a 1.0 tog... Although they dont recommend putting bubs in Grobags until at least 6 weeks because it is better to wrap them in those first few weeks so they feel more secure. So by the time your bubby is 6 weeks it will be halfway through October and by that stage it is getting a lot warmer weather-wise... but then in the really hot part of summer 1.0 tog would be too warm IMO, but then you can easily just put them to bed wearing not much at all.... and then come Autumn of the next year, that same grobag is still likely to fit (the infant size usually fits most babies up to at least 6 months old, sometimes a fair bit longer depending on how long your baby is)
Also, maybe see what someone else from Melbourne says, as I'm not too familiar with your weather down there.
