These days electric buses come in a number of shapes and forms. Trolley buses are the most familiar and gain their electricity from the overhead lines. China still builds them, and at a remarkably cheap price and fair quality. They are still made in Europe and by importing key components, they could be built here. The disadvantage is that the overhead wires and and associated infrastructure is expensive and not very attractive. The buses normally have to stick to the route of the overhead wires, although there are hybrids that allow them to go beyond the wires.
Christchurch had a fleet of Battery Electric buses. From a passenger perspective, they seemed pretty good. Performance is limitied by the capacity and condition of the batteries. If these buses are used on hills, it seems likely that they'd require a lengthy mid-day recharge.
Shanghai has a large fleet of "Capacitor Buses". The capacitors act as batteries but can be charged very quickly. At each terminus, the pantograph extends up to the short section of overhead infrastructure and replenishes the capacitors in just a couple of minutes, and soon, the bus is ready to undertake it's next cross-town journey. Personally I think they'd work well in Dunedin as a large part of the urban network could be operated from a small number of charging hubs.
There are also hybrid buses that use small conventional diesel engines or even jet engines running continuously to replenish the batteries. When the bus is going up hill, the bus is able to draw more energy than the engine is producing and on the flat the batteries can recover. They don't require any infrastructure but do use diesel / fuel oil.