The best time to visit the Central Queensland Gemfields is undoubtedly in winter - when it does get cold at night, and the days start out warm - but you'll be stripped back to short sleeves and less for males, by lunchtime. It is still warm when the sun is out.
Winter is ideal - or general rule of thumb is March to October.
The locals will tell you that the best time to come to the gemfields is after the wet season, about May - when the ground is wet and softer and new pickings have been exposed. You will see the locals emerging in their Drizabones and hats after the rain specking over the wetted ground.
If you do go out to one of the bigger fields for a fossick, be mindful of the fact that there are plenty of old shafts and tunnels still there. Plenty of sprained ankles and big frights from careless trudging across the fields.