What state/area? Best trick I picked up was to check the back of their neck. You want warm, not hot (or sweating) and not cold. Mottled skin can indicate they are cold or poor blood circulation. In general, you want about 1 layer more than what you're wearing. In saying that we go off me, not hubby, because he would happily be mostly naked while I have a jumper on. Easiest thing is to have the temp temp controlled with split system air con but not always possible. You want to dress for the coldest part of the night because it's easier to remove a blanket/pants/socks than to add clothing layers while a baby/child is asleep. It's a bit trial and error too, but you get used to what is best for them at x temp. Right now it's apparently 20 but feels like 24 in Brisbane. I have my bub in a bonds long sleeve onsie with a stretchy wrap and then another stretchy wrap on top tucked in short sheeted as a blanket. I was using a thin sleeping bag but it's a little cooler now it's been raining so the wrap is warmer.
@Kirra I’m in Melbourne so we had it easy the first few days but now we’re in for 35 degree days. I just had her in a long arm but no legs and muslin wrap and when I woke her she was 37.1 (checked just out of curiosity) is that alright or too hot. This is so stressful ahah 😅
Honestly, I've never really checked their temp as a means of checking if they're overheating or cold. In saying that, very low 37 is a normal temp. If it's super hot you could go a singlet and nappy but air flow helps too. You don't want a fan right on them if possible, but in the room to circulate air. I've walked bub to school a couple of times and it's not super hot just yet but warm... maybe 30 degree days and I assume hotter in the sun. I had bub in a thin long sleeve but no leg onsie and with a fan in the pram aimed above his head. It's really annoying until you get the hang of guessing what might suit them, and even then, each baby is different! #2/3 for me were twins and one preferred to be dressed warmer than the other. So anything people tell you is kind of generalised, and you've got to work out what suits yours.
I don't know what the rules are because they change every month, but my last baby was "dress them 1 layer more than what you're wearing." But I also went off the house I was in. Our room always got the setting sun so it was always hot and stuffy at the end of the day so I'd put him to sleep in a singlet and then when he woke up for feeds, I'd see if it had cooled down enough to chuck a onesie on him.