Normanby-St Clair is Dunedin's flagship bus route, with the most frequent service and highest number of passengers. It is a commercial route operated by Go Bus Transport and they don't have to follow any rules of the Otago Regional Council. This is why they sometimes run quite old buses. They can change the fares too if they want (but they're the same at the mo).
At night and on Saturdays Go Bus cuts the route down to half-hourly and then hourly as the passenger numbers drop off. On Sundays or public holidays, there's much less money to be made from selling tickets so the bus route becomes an Otago Regional Council-subsidised route. The ORC contracts to whoever is cheapest, so at the moment Go Bus runs it as an ORC contractor, but it could get contracted to any other bus company at any time. They DO have to use ORC-approved buses and ORC standard fares on Sundays and public holidays.