Manuka (mah nu ka, in Oz pronunciation) is the local NZ name for Leptospermum scoparium, as mentioned above. Leptospermum is in the Myrtaceae (along with eucalypts, Angophora, Darwinia etc) and L. scoparium also is found in Australia. Most Myrtaceae have considerable quantities of 'essential oils' in their tissues, especially the leaves and it is these oils that give the infusions their flavour and the bushes their extreme flammability.
Honey with large quantities of myrtaceous pollen seems to have the topical antibiotic ("topical" as in putting it on the wound rather than just swallowing it) effect irrespective of species, but seems to be stronger in honey from L scoparium, L. laevigatum (coastal teatree) and L. squarrosa (swamp paperbark); the latter two are Oz rather than NZ species.
But more research would tie the effect down to contributing causes much more specifically.
Yellow box is a very smooth flavour on the tongue and is widely sold but much of it has a lot of other sources as well. The current (Oz) drought has severly affected apiarists and honey production so conserve your stocks.