Best to see a tax agent or ATO's technical advice
This might be a scrip-for-scrip rollover. Doesn't look like it would be a partial scrip-for-scrip rollover, as there doesn't seem to be any cash proceeds. So the total cost base of the AUI shares received for holding DUI shares, would be the total cost base you had for DUI shares before being delisted
https://www.ato.gov.au/forms-and-instructions/guide-to-capital-gains-tax-2025/about-capital-gains-tax/investments-in-shares-and-units#ato-Scripforscriprollover
https://www.ato.gov.au/forms-and-instructions/guide-to-capital-gains-tax-2025/appendixes/appendix-4-definitions#ato-Scripforscriprollover
As for filling out 2026 tax return
I think the proper way is to tick "Australian interest, or other Australian income or losses from investments or property" in personalisation to get the capital gain section
Don't include any capital gain/losses for the DUI > AUI share conversion
If this is this is the only CGT code used, then use the code "[Removed by moderator] Scrip for scrip roll-over (Subdivision 124-M)" for the "CGT exemption, roll-over or additional discount type code"