Te Rangatahi, Advanced 1 and Te Rangatahi, Advanced 2
These books may seem a little dated, but I enjoyed them thoroughly and they their lessons were my foundation. There are no online audio resources, audio files and such, but the first book is what really sucked me into te reo. Each chapter has a story that utilizes the words and grammar structures of the language. It took me several months to work through the chapters, which is probably pretty fast for a book of this density, but my background with ‘ōlelo Hawai‘i certainly helped.
A Māori Reference Grammar, by Ray Harlow
If you are a language geek like me, this is a great help. I have some (but not a lot) of linguistic training, so some of it does go over my head. But still a worthwhile investment.