Homeowners looking to rent their cars or charge visitors for pick-up and drop-off services during the Rugby World Cup will be breaking the law, warns the New Zealand Transport Agency.

Some homeowners who have listed their homes for rent on websites will offer the use of their cars for an extra fee and chauffeur visitors to and from the airport and rugby games for a fee.

But John Doesburg of the NZTA says charging for the use of a car is illegal unless the vehicle owner holds a transport service licence.

He says the vehicle also needs a Certificate of Fitness, not just a Warrant of Fitness.

That applies to renting a private car and charging someone a fee to drive them. The car owner also needs a Passenger Endorsement on their driver's licence.

Those caught breaking the regulations face up to three months' prison or a $4500 fine.

If the use of the car is included in the house rental and pick-up/drop-off services are included at no extra cost, Doesburg says the regulations do not apply and a transport service licence is not needed.