Thanks mate... i am sure they are a rock solid battery :)

Turns out the missus rang Battery World

They recommend the Varta (which i believe is the OEM battery)
585 015 Varta Black Dynamic Car Battery 12V 85Ah (Short Code: F5)(Varta DIN: 585 015 076) - Car Batteries - Varta Car Batteries

They quoted $375 AUD

There is also a slightly bigger DIN100 Varta, but sadly its 20mm higher, so it will not fit :(
https://carpartsconnexion.webasyst.net/shop/product/g3/

Happy to hear about the CAT battery tho Smokey.... But if the missus needs to get a Battery in hurry, she might have to go to Battery World (good, i don't have to pay then)