I bicycled between Cairns and Darwin in July and August. It was cool at night until I reached the Katherine area. It went up to low 30C during the day at that time of year but at night it could get to 10C in the Atherton Tablelands. Near the coast the temps are moderate with no extreme temps.

On the way (near Borroloola) I met a fellow who bicycled "up" from Melbourne earlier in the year, and a fellow who left Hobart a couple of months earlier - BUT he was doing at least 120 kim a day when he was on the road.

There are no 50C temps in that region even during the wet season. Mind you, inland it does get warm and you will need to be careful that you take enough water. OTOH, you can always flag someone down and ask for a couple of liters of water - most folks are quite accommodating. Some folks asked me to join them for tea and biscuits and some even offered beer, or chocolate bars and "energy drinks".