Great Barrier has a Navajo/Cheiftain that sits in Kaitaia all day...
On existing fares, a quick back of the envelope figures, 4 pax each way would cover Kaitaia to Auckland in a Caravan. Loads would be around 1000kgs from Auckland and 1150kgs to Auckland. (No JetA1 in KT)
Depending on your overheads, 5-6 would turn a profit.
I don't know what Sunairs plan would be for a 12 seater. It could be a Caravan with the STC to carry 12. But like I said, 1000kgs out of Auckland with 20kg of baggage each, you would be planning on pax being 63kg.... Now I don't want to tar everyone with the same brush, but a lot of the pax leaving KT are heavier than 63kg.
A figure of 9 pax would be more realistic.
A PC-12 would be similarly hard pressed and usually only carry 9 pax and at a much higher cost than a Van.
A King Air would be hard to turn a decent profit as would most twins.