Being a van it was actually built at the Elizabeth (Adelaide) body plant and assembled at the Pagewood (Sydney) assembly plant. Most Holdens of the day were built from internal orders initially as unallocated orders, and then:
1. Dealers sold them off the schedule (cars not yet built but become "allocated" on the schedule once the dealer assigns it); or
2. Dealers sold them out of unallocated stock (complete cars ready to be sold held in storage); or
3. Dealers were allocated or themselves ordered cars out off the rolling schedule and had them in stock ready to sell to walk in buyers; or
4. Dealers would sell cars in stock at other dealers and do a wholesale deal or dealer swap with the dealer that had the car.

Of course there were a few other methods, one of which was a customer wanted to order a vehicle that was not one GMH built for unallocated stock, these were "special order" cars. They were either heavily optioned cars (my HJ Prem is like this), special or unusual or fleet colours eg Tuxedo black HQ Sandman or abnormal driveline eg 350 HQ ute, 202 HQ Statesman after 1972.

None of this is any different to today either. I've bought 2 x new Holdens in the last 10 years or so by two of these methods. I bought a white VSIII V6 manual S pack ute in December 2000 by method 4, and I also bought a white VY SS manual ute in early 2003 by special order as I wanted black leather seats (took 5 months to build as it got put on the rolling schedule).

So back to the question, for a Pagewood assembled van to end up first registered in South Australia it is not very common, but can happen , normally by method 4 or sometimes method 2. As Innuendo says there were also periods when one of the body plants wasn't building a certain body style meaning they had to be sourced from another zone office's stock. There are other possibilities too, but the above is the most likely.