I got the tag end of a program on the radio a few years ago about how the New Zealand accent developed, and it is an entirely vowel-shifted one. Australian largely is, too, but less pronounced.

The best advice is from people who make drama TV outside the US for US syndication because they have to work with locals. Renaissance Pictures did this for Hercules and Xena and there was a brief chapter in one of the merchandising books explaining what they taught (both shows were filmed in NZ and Lucy Lawless is actually a Kiwi).

I also saw more recently a very interesting program about how English developed. There are people today who can and do speak Middle English from before The Great Vowel Shift. It sounds odd to our ears and is much more Baltic-sounding, but fascinating nonetheless.

Wade.