New Zealand, unlike the UK (and indeed most countries) prohibits taxis from using bus lanes. One reason for this has been because (unlike most countries) New Zealand's free market approach to taxis means there are more than most, and cheaper as a result.
Nevertheless, wherever a bus lane operates significantly under capacity, then other public transport should be entitled to use it - that means taxis. Taxis don't compete with car usage. Most taxi users are either without ready access to a vehicle, without access to a car park, have no viable public transport alternative or are drunk! Allowing taxis to use bus lanes would save taxi users a fortune, and taxi drivers could undertake more trips, and by removing them from parallel lanes would help cut congestion more generally.
Of course to do this would mean removing the ideological commitment to buses, which themselves need to carry eight passengers to be a better use of road space than a car, and twice as much as that to be more environmentally friendly. If local authorities were more committed to reducing congestion rather than simply encouraging use of public transport, then they might actually support this. Remember that almost all bus lanes in London allow taxi usage.