Niel: No one has mentioned LPG heaters, so here goes for completeness. They produce a huge amount of heat both radiant and convection. Great solution, but not for NZ. The by product is water and lots of it. In dry countries that is fine, but not here. Would love to see someone develop an LPG heater which captures the moisture. Yes you can vent it outside, but there goes lots of heat as well. (More about moisture down below.)
Carbon monoxide too, these standalone LPG heaters are dangerous in smaller rooms.
I have a vented gas heater -- the expelled air is very hot and a total waste.
I've investigated into heat exchangers a little, to see if the heat can be extracted from the vented gas and reintroduced into the house. Such a waste!