Thanks everyone for the input.
Sadly it seems our plans have hit a few spanners in the works.
Molesworth/Acheron is closed at the Clarent river bridge, so cannot be used as a through road from Hanmer to Picton.
As for the Rainbow Road, I contacted the farm to ask about early access and they said that due to heavy rain and slips access is unlikely for quite a while.
So it looks like there’s no viable way to head north on the dirt.