The QDO board asked us to investigate what happens when an unroadworthy registered motorbike/tractor is in an accident.
As you may be aware limited use registration does not require a safety certificate / roadworthy. Instead, the vehicle owner signs a form to verify that the vehicle complies with the vehicle standards of construction.
The Compulsory Third Party (CTP) insurance is applied and will cover you for personal injury claims made against you by other road users including drivers, passengers, pedestrians, cyclists and motorcyclists if a crash occurs (subject to the scope of the Motor Accident Insurance Act 1994).
CTP insurance will not cover you for property damage.
The Department of Transport and Main Roads says that in some circumstances, in addition to CTP insurance, you should also obtain a public liability or general insurance policy. This information is publicly available on their website. https://www.qld.gov.au/transport/registration/register/conditional/about-conditional
Given that the owner has signed instead of getting a safety certificate it is likely the onus will be on them if an incident occurs, similar to workplace health and safety compliance requirements with vehicles being used on the farm.
You should keep the vehicles in a "safe" condition and keep a maintenance record. The maintenance record is important. If you find yourself in an investigation it can be used to prove in court that you have done all that is reasonably practical and the accident was not caused from something that wasn't working - lights, indicators, brakes, tyres and so on.
If you operate, or if you own the vehicle and it is operated by someone else, please talk with your insurance provider or broker for more information to ensure you have the level of cover you need.