The half circles cut out of leaves is the leaf cutter bee. Your description of perfect little half circles is very apt. They almost always produce very neat cuts. These are native bees (stingless) and they are taking the pieces of leaf away to make a nest for their larvae but they are solitary (just 1 female working alone on a nest just for her eggs) so they do not produce honey.
http://www.aussiebee.com.au/leafcutter_bee.html
I'd be reluctant to kill them off, just because they are fascinating native insects. The few bits of leaf they take might be a bit unsightly but won't usually harm the tree in the long term.
The tiny white flying dots are probably whitefly. both young and adults suck sap from the leaves and can sometimes cause damage if the numbers are high. They do not seem to like water so hose them off with a spray a couple of times a day can reduce numbers a bit. They are attracted to yellow so you can get sticky yellow traps to catch them. Most insecticides will reduce the numbers but they do hatch out very quickly and numbers will build up again after the first spray and may need to treat again. Try Natrasoap, or eco-oil for low toxic solution. Confidor is more toxic and will kill them. Confidor will also kill the leafcutter bees and probably all the good bugs like ladybirds and honeybees in your garden as well.