Stumpy
It's not the weight, but the number of seats (more than 9) that mean you need 2 pilots on this type of B1900 operation. The 1900 is certified as a single or two-pilot aircraft. There is no way a passenger would have been let sit in a pilot seat on the operation in question (which I'm pretty sure would have been a scheduled Air NZ service). As mattyj said, he would have been in the front passenger seat.
As far as I'm aware, Vincent have all but pulled out of their Australian operations due to the huge (and increasing) demand for their services back home (buy Eagle).