"Kia Ora" is not a standard Maori thank you, it's a standard greeting/farewell. It is listed in many new Zealand dictionaries and glossaries; here is one of the better explanations (with usage examples) that I've seen:
An Argentine-American in New Zealand: NZ Dictionary entry
A sound file for pronunciation can be found here:
Reo Maaori Sound Files
Non-Maori Kiwis use this expression, too. I've received many emails from my NZ friends that began with "Kia ora".
Hope this helps.