100W should be OK just make sure that you check it with a good thermometer once it's up and running. The hardest thing that i find when heating all glass enclosures (especially at this time of year) is getting it to heat UP enough.
I am using a 100W MVB in a 3 ft fish tank with hood for my henrylawsoni. This setup works well for this time of year but would cook the tank come summer unless the top was comletely ventilated.
Bluey, I dont know what you are keeping but you could use a 160 watter in that if it is a glass tank and you dont live in Darwin. Hornet, Gladstone is a pretty warm place so your 100 watter should be fine for tawneys.
try to copy what is outside in their range mate. it would max out at 25 for a few months yet. Also copy the daylight hours. I.e. dont have the daylight globe on as long in winter. All I do for my dragons (that are inside) is use the heat light during daylight hours. In winter the cage is cooler because it is cooler outside. And I havea big fat frilly in there right now.