Taumarunui was apparently "immortalised" about being on the main trunk line by Pete Cape's folk song. (http://folksong.org.nz/taumarunui/index.html) however there was a time when it also had a non-scheduled air service operated by Eagle Air to and from both Hamilton and New Plymouth that was operated, as required, as part of the Hamilton-New Plymouth route. Unfortunately the Eagle Air timetable of it is undated so I have no idea when the service started, ended or if it operated very often. I would be surprised if the service would not have been a real winner.
As an aside, Taumarunui also had a railcar service to and from New Plymouth that connected with the north and south bound express trains/railcars from Auckland to Wellington that went through Whangamomona. The line recently experienced sever damage after a derailment and Kiwirail has moth balled it pending a decision on its fate.
If you know anything more about the air service to Taumarunui please drop me a line at firstname.lastname@example.org