I haven't been able to find another link to the Privy Council decision. The easiest method might be to visit the library at the local law school. It's in a set of volumes called "Appeal Cases".
Mahon's report is probably out of print by now, but there are several copies available at the Auckland Public Library. They also have the transcript of the evidence given at the hearing (but not the documents).
There is also a useful book called "The Erebus Papers" (Stuart Macfarlane - Avon Press - 1991). It has decent photographic reproductions of many of the documents and also has extracts from the transcript of evidence. Unfortunately, the book also has the opinions of the lawyer who wrote it. Bit if those are ignored, it is very helpful.
Incidentally, if ALPA are going to do are proper job on the proposed website, why don't they dig up everything, scan it, and put it on the site so that everyone has all the information?