Thanks Veritas. Yeah definitely will double check with the ATO...

Thanks Veritas. Yeah definitely will double check with the ATO and\or my tax agent if I proceed with it. The current online content on the ATO website also indicates the same such as when filling out a Bas statement. You treat shares as financial supplies and in G11 you can state brokerage, then in 1B you can enter the GST of your brokerage (because it falls under the exceptions of Reduced Credit Acquisition).
https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst/in-detail/your-industry/financial-services-and-insurance/gst-and-financial-supplies/reporting-financial-supplies-on-your-bas

Also the fact that they haven't archived the post and keeps it active means it could still apply. It seems to be something that has existed for a while because I first found out about it on a post which is even older on another forum.

However there seems to be a newer post which indicates you can claim the full 100% if you meet certain criteria:

https://community.ato.gov.au/s/question/a0J9s000000TXraEAG/p00248027?referrer=a0N9s000000DacIEAS