NB: There's already a project that does this, and includes other countries:

• native-land.ca

The idea is to create an 'overlay' for OpenStreetMap that would show the nations of Australia.

I have no idea how to do this.

As best I can tell the boundaries on the maps are not, and are not able to be(?), perfectly accurate.
see: https://aiatsis.gov.au/aboriginal-studies-press/products/aiatsis-map-indigenous-australia

Considering that these borders change over time, it's not really possible to make an accurate map.

David R Horton is the author of the AIATSIS map, the latest version is from 1996.

• https://www.spatialsource.com.au/gis-data/4-maps-uncovering-aboriginal-history-culture
• http://www.nationalunitygovernment.org/pdf/aboriginal-australia-map.pdf
• http://ourlanguages.org.au/category/resources/maps/
  • http://ourlanguages.org.au/indigenous-language-map/ - someone done a GMaps one, but it seems to be gone?
• http://nationalunitygovernment.org/pdf/boundaries-map.pdf - good one, shows more landmarks
• https://c21ch.newcastle.edu.au/colonialmassacres/map.php
• https://www.deadlystory.com/page/aboriginal-country-map
• https://mgnsw.org.au/wp-content/uploads/2019/01/map_col_highres.jpg
• https://www.aboriginalvictoria.vic.gov.au/aboriginal-places-and-objects
• https://catalogue.nla.gov.au/Record/3642040
• https://web.archive.org/web/20160426141016/http://www.abc.net.au/indigenous/map/

• https://mashable.com/article/indigenous-map-america/

• https://wiki.openstreetmap.org/wiki/Tag:boundary%3Daboriginal_lands

• https://press-files.anu.edu.au/downloads/press/p73361/pdf/ch0648.pdf
• https://en.wikipedia.org/wiki/List_of_Indigenous_Australian_group_names
• https://datagovau.ideascale.com/a/dtd/Shapefile-for-Aboriginal-Australia-Map/33804-26233 - person looking for map data 'shapefile'
• https://en.wikipedia.org/wiki/Encyclopaedia_of_Aboriginal_Australia
• https://catalogue.nla.gov.au/Record/7760724