Hi Alycia
When I had a serious infection recently I was told that I needed to get to emergency if I hit 38.5 so 42 is definitely high! For children I believe they’re even more conservative in when they recommend getting medical advice.
I’m not sure if you’re just asking for curiosity or not but I would say if you have a child with a fever that you’re not sure about I would recommend giving the afterhours GP Help Line a call which is free. I’ve used it before and they saved me a trip to the hospital post-surgery and my friends with young children call it quite a bit. They will assess the risk on the phone and let you know what to do. Depending on the situation they either just put you in touch with a registered nurse or may direct your call to a GP.
You can reach them on 1800 022 222 and there’s more info here – http://www.healthdirect.gov.au/after-hours-gp-helpline
They also have some resources to read up on about this kind of thing that are much more informative than I could possibly be –
http://www.healthdirect.gov.au/fever