CaveRock, Orcon, Maxnet ... ? Great ISPs in general but try to sell the idea of using a little company to a corporate who spend 30k plus a month on telecommunications
I'm pretty sure that Orcon, Caverock and Maxnet have customers paying over 30k per month, Caverock which is in Christchurch (looks to be part of Snap) on a traceroute provide Canterbury Uni with their internet and have their own fibre, and they will be paying a bit for that.
If your customer is paying over 30k then surely they will have an Account Manager that would sort that kind of thing out!
I dont know why with all the information we know about Xtra people would actually still use them!
I would personally go with the ISP with the most upstream Tier 1 suppliers, that is closest to your location (especially with UBS, as we all know about UBS backhaul) there is a good map here http://www.ispmap.co.nz/topmap.html