There are downloadable Maori fonts for public use.
Can get more info here or (less to read) here.
I usually just use the Alt key and a bunch of numbers.
How do you know which bunch of numbers to use? Get them off the Character Map.
Use the search function on your machine to find the Character map. Usually in the 'Start' menu on your Desktop or in the Accessories Menu - Start/All Programs/Accessories/System Tools/Character Map
Click the letter and type of macron you want in the grid. In the bottom right corner of the Character Map, there'll be the Keystroke information.
You see here, the Keystroke is the 'Alt' button plus the numbers 0225. Hold down the 'Alt' key, type in your numbers and then release the 'Alt' key.
If you're too lazy to keep looking them up, here's the guff you need to know. Write it out somewhere and leave near your machine or pin to the wall or something.