I am Wellington based and was new to the Central Plateau area. I needed transportation to Whakapapa as I planned on climbing Mt Ruapehu. One of the local providers quoted a price of $120 for the trip...luckily I shopped around as Dempsey quoted 1/3 the price and got me there without any drama. the driver, Richard, was very knowledgeable and enthusiastic, bringing up the energy on the bus and sharing the local facts and knowledge.
At the end of the day, one of the Tongariro crossing passengers did not come back on time - Richard ran up a part of the crossing, a few kilometres on a steep uphill, to find the passenger. Unfortunately she injured her ankle and needed to be flown out. I have completed the crossing myself the day before and remembered another provider stating that those who do not show up on time will be left behind and have to pay for an additional shuttle to pick them up.
There are many shuttle providers in the National Park area, and there is little price differentiation...what counts is the quality of the service, and from my experience Dempsey will provide a unique experience while treating their guests like human being, not clueless ATMs...