The Automobile Association has raised misgivings about safety standards covering agricultural vehicles, as a result of new transport rules that take effect from June.
The changes mean that slow moving tractors and hay bailers that travel below 40 kilometres per hour will not require a warrant of fitness.
Federated Farmers said the rule change was practical as fewer rural garages were able to do warrants of fitness.
Under the changes agricultural vehicles will also be exempt from work time requirements that limit the hours an operator can drive without taking a break.
The speed limit tolerance will be raised by 10 kilometres per hour.
The Automobile Association is worried that the changes could lower safety standards.
