Quick update for any who has a similar cistern. This model of outlet valve (https://www.bunnings.com.au/fix-a-loo-thunderflush-dual-flush-cistern-outl ...

Thank you all for you assistance. I’ll buy this replacement and see if it works and update this tread with progress!

Thanks for your advice. We will consult an engineer in this case and update you on an outcome.

Easy to remove the door, then the mesh, and spray paint to a colour of your liking. If you want a tradie to do this Airtasker might be a good place to ...