There was a recent post on here where someone (not in WA) had successfully had there ATM increased by contacting Jayco service who issued a new compliance plate with a larger payload without any upgrades. I tried it on with The service manager at Caravanland and this is the reply I got.
Good morning
Thank you for your email regarding increasing your atm, unfortunately this is not something that we carry out here or can advise on the best place to start is with the department of transport list of engineers certified to sign off found below. They will advise what is needed to carry this out and issue paperwork for transport to update your atm