Hi Simon,
I've asked around and unfortunately there is no way we know of to extrapolate the page number from the metadata we have in the DigitalNZ API.
As you're probably aware the whole Papers Past site recently had a big over haul. The metadata in DigitalNZs API for Papers Past is also overdue for a similar update.
We are hoping to refresh our Papers Past metadata in the new year, including getting up to date on the considerable amount of new papers past content that we don't currently have in the DigitalNZ API.
Hopefully we will also be able to the bring these page numbers into the metadata when get to this update.
Let us know if theres anything else we might be able to help you with.
Thanks
Dan