If you are not using this already, then I recommend that you download the following software from Microsoft to ensure that you insert the correct macron character e.g. ā, line over a vowel and not some other character e.g. ä, the umlaut, or double dot over the vowel, which doesn’t designate a true macron.
The following information has been provided by MS on their site…
“This download reconfigures the US/International keyboards supplied in New Zealand to support the macron characters used in the Māori language”. “After installing the Māori Keyboard definition a user can press the [~`] key once then any vowel. This applies to upper case as well as lower case letters. If a user requires the left quotation the user simply presses the key twice. If the user needs to use the tilde character, they simply use the traditional method of [Shift]+[~`]. i.e. the ā character would be entered by pressing the [~`] key then the [a] key.” Close quotes.
Download software here…Māori Keyboard
You can also read about macron conventions in an article about Māori orthography written by Te Taurawhiri i te Reo Māori.