This is one of the best secondhand bookshops in Auckland for NZ books:
http://www.hardtofind.co.nz/
I'd send them an email and ask them to keep a lookout. They definitely had the Mahan and MacFarlane books the last time I was in there about a year ago.
And here's what's available on Trade Me at present:
http://www.trademe.co.nz/Browse/Sear...earch&x=18&y=8