There is a problem with retrieving certain sets.
If I search within the "Digital NZ Sets" primary collection using the search API, or using the DigitalNZ website (which I assume is using the same API), I get a list of sets, e.g. from the website:
... but if I try to click through and look at a particular Set, I get a 404 error. e.g. the first one in the list, "Geoff Robinson Retires":
Oh dear.
We haven't been able to connect you to the page you are looking for.