As a Brit I find the NZ English pretty much the same and easy to follow.
Over here kebab means the same thing as in NZ. But we usually specify, doner or shish if really pushed. But if someone says they ate a kebab last night, my brain would picture a doner, as its the most popular
Most kebab houses don't really go in for shish kebabs.
TBH, I've not been overly bothered by the minor changes they have made to their speech in the acting story part of the show. I understand both just fine as we have enough US stuff on TV and in cinemas.
It does seem a shame they changed some wording in the songs though. Pencils In The Wind sounds so much better as a title and the use of the word Sellotape works better imo. As for others, I don't really notice as I only ever watch each episode once, so it doesn't have time to grate on me. Sellotape was sorely missed as it is one of my two favourites of their's. The other being Busdrivers Song.
Don't forget though that this show is being sold round the world. It begins airing in the UK mid September and New Zealand also. Both of those will notice the Americanisms. Especially the Kiwi's who'll probably notice the changes in their day to day speech in the show more than the songs. To my ear all the Kiwi's in the show have toned down their accents for US audiences. Apart from when they use word pronounciation as a joke in the show, like 'dead/did' and say six when meaning sex.
Ummm all that didn't help any