Go and talk to an authorised engineer. They'll be able to tell you if it legally requires engineering based on Vehicle Standards (VSI) according to their interpretation of the rules
However, only a licensed roadworthy inspector is qualified to tell you, they are checking your vehicle on behalf of your state registration body. Their job is to ensure that your vehicle fits within all relevant rules and restrictions, and that it is fit for road use.
If you front up to a roadworthy inspector and they say "no, sorry, it needs to be engineered", then yes, you need to get it engineered. If they say "no, it's fine", then it doesn't need to be engineered.
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