common length today is the 16/70 with a 70 mm long case
https://www.sellier-bellot.cz/en/products/shotgun-shells/list/12/

but the 16/67,5 is also made to be used in any 65 mm long chamber

https://www.sellier-bellot.cz/en/products/shotgun-shells/list/10/

I see the czech make now also 65 mm long shells again but this is new to me

https://www.frankonia.de/p/sellier-bello...ategoryId=62235

the 16/67,5 is available with steel and bismuth pellets

https://www.frankonia.de/p/fob/16-67-5-z...ategoryId=62235

another option would be the italians

https://www.baschieri-pellagri.com/it/catalogo/cartucce+da+caccia/1/1/16.aspx

the short shell is called here 67 mm long but its the same. because the plastic shell is much thinner than a paper shell they made it 67 or 67,5 mm long so its more space with a star crimp and still safe enough.

fiocchi have also the 67, smm long shells

https://fiocchi.com/en/catalog/ammo/shotshells/16-gauge.html

you better ask the australian importer for this firms directly.