I was reading the Proto-Indo-European mythology Wikipedia page when I started to wonder if there were any similar reconstructions for other related groups of mythology. I'm particularly curious if there's anything for Aboriginal Australians or Polynesians although I'd like to learn about any that exist, and if there are what would be some good sources to read about them? Even some good sources to read more about Proto-Indo-European mythology would be appreciated
Reconstruction of Proto-Polynesian mythology
It seems that attempts at reconstructing an overall Polynesian mythology are not as abundant and convincing as in the case of Proto-Indo-European. However, there are many articles which use the comparative approach to study specific mythological themes and motifs. For example, in The First-Order Anthropomorphic Gods of Polynesia and Was There an Early Polynesian ‘Sky Father’? Jeff Marck compares recurring themes in Polynesian religions. Also, in the first section of both articles he gives an overview on previous reconstructions and comparative studies on Polynesian mythologies which you might find useful for further reading.
On Australian mythology, Michael Witzel in section 5.3.2 of his book The Origins of the World's Mythologies (2013), gives a nice overview on the similarities and differences between various Australian mythologies. However, regarding a reconstructed pan-Australian mythology he comments:
[...] have the Australians just preserved a late stage of Pan-Gaean mythology, which already had, for example, a world tree? To answer this question would depend on a secure dating of the (so far not yet) reconstructed pan-Australian mythology [...]
Also it should be noted that, while ambitious (and certainly useful in some respects), the book is quite controversial. See for exemple Bruce Lincoln's review.
Further reading on Proto-Indo-European mythology
In my opinion, the best starting point on this subject is Jaan Puhvel's Comparative Mythology. It's a wonderful introduction to comparative Indo-European mythology. Other recommended reads are definitely Indo-European Poetry and Myth by M. L. West and Death, War, and Sacrifice by Bruce Lincoln.
If you are particularly interested in the reconstructed Proto-Indo-European pantheon, Peter Jackson's article Light from Distant Asterisks. Towards a Description of the Indo-European Religious Heritage is a must read.