How similar are Hawaiian and Maori?
Generally very similar. A linguist would have to go into the specifics of language families. What I can tell you is that culturally, Kanaka Hawaii and Māori are considered siblings, with Kanaka Hawaii being the older sibling and Māori being the younger. I can tell you that there are sentence patterns and words that are exactly the same in both languages, albeit with some sound changes, but to proficient speakers, the sound changes are not very difficult to interpret. There is a wealth of vocabulary unique to each, but at a base-level; still very similar linguistically.
There is great similarity in the grammar and vocabulary, but there are some (fairly consistent) spelling and pronunciation changes. I would say the differences are probably greater than the difference between standard American English and RP British English, but probably not as great as the differences between American English and Scots.
For example for many words, the same root is used in both languages but the sounds are made further forward in Maori and further back in Hawaiian. So words might be spelled/pronounced with t in Maori and k in Hawaiian, with k in Maori, but ‘ in Hawaiian, and with r in Maori, but l in Hawaiian. So "aroha" in Maori, but "aloha" in Hawaiian. "Tokerau" in Maori is "ko‘olau" in Hawaiian. And "taro" in Maori (English got its name for this plant from Tahitian which is even closer to Maori), but "kalo" in Hawaiian.
The Ni'ihau dialect of Hawaiian also has the t/k difference, so tangata Maori and tanata Ni'ihau probably understand each other even more easily.
Māori is more complex phonetically speaking than Hawaiian, although both are languages with few phonemes. The grammar of both seem rather similar. Hawaiian might be slightly more likely to be simple, generally speaking. You can compare them using the links below. They are probably as similar to one another as Romanian is to Latin (in other words, very similar but definitely not impossible to tell apart, even to untrained eyes and ears).