After realising my guide data hadn't updated in the last week, this post https://www.geekzone.co.nz/forums.asp?forumid=126&topicid=225606 mentioned that we should be using EIT instead of MHEG5.
So I changed the collection parameters to EIT, and this produced 7 days of guide data for every channel except TVNZ 1, 2, +1 and +2. It seems it's only getting 2 entries for those channels (now/next)
I'm in Auckland using Waiatarua 538 MHz. Any help appreciated, cheers.