Auckland Libraries has several books on the subject of understanding and managing hoarding behaviours:
Books about hoarding behaviours that you can borrow
The Taranaki DHB have written an interesting document a few years ago:
Hoarding And Squalor Guidelines
The Southern DHB wrote something very similar:
Good Living Conditions Southern
This UK public Facebook group shares useful information and articles for people with hoarding behaviours, professionals who help them and family and friends.
https://www.facebook.com/HoardingDisordersUK/
Articles
Other Resources:
Psychology Today NZ – Hoarding Disorder Enter your postcode in the box on the right to find therapists who can help with hoarding behaviours.