Melbourne: Installation of tow hitch void warranty

If we assume that Melbourne is in Australia, then you need to read your automotive consumer protection law.
Consumer guarantees | ACCC


Here in the US, the Magnuson-Moss Warranty act defines our rights. Magnuson–Moss Warranty Act - Wikipedia, the free encyclopedia

Toyota in Japan will not have much impact on your local situation.

 

From the dealers standpoint, they have no way of knowing what you are doing with the hitch. There seem to be a lot of owners that ignore the owners manual and tow various trailers with their car. It will be interesting to see what happens when the CVT fails.

 

That doesn't sound like an official statement. Toyota are somewhat more liberal about modifications than some of their dealers realise.

While Australia does not have anything quite as explicit as the Magnuson-Moss Act, the dealer can't just declare your warranty void because of something they don't like. I have an engine pre-heater and grill block and there's plenty of Prii in Australia with plug-in conversions and modified ECU that are still covered by Toyota warranty without any fuss at all. I think that your dealer is talking out of their arse.
Of course, if it's a Hayman-Reese with 3000kg capacity or you drove in with a grey nomad's caravan, then they have cause to wag their finger and perhaps void the warranty.

Based on the specs, you have 440kg to play with (GVM - Kerb weight). So here's how to stay on the right side:
Less 45kg for a full tank of fuel
Less x kg for each passenger
Less x kg of luggage
Less x kg for hitch and bicycles.
As long as the result is greater than 0, you are not exceeding the GVM.

There's no figure for the maximum towball weight since Toyota will not supply a towbar for the Prius. However, I couldn't see more than 75kg for towbar, rack and a bunch of bikes.

I wonder if there are any light towbars that meet the Australian Standard
Toyota Prius specifications Prius

• Note: Kerb, tare & payload weights are nominal and will vary depending on options and tray/body type. Gross Vehicle Mass (GVM) is the total permissible combined weight of the vehicle, including occupants, fuel and cargo.Weights
  • - Kerb weight (kg)1365
  • - Gross Vehicle Mass (kg)1805

And just because it says, "Note: Towing capacity subject to regulatory requirements, towbar design, vehicle design and towing equipment limitations. Ask your Dealer for details of Toyota Genuine Towbar capacity and availability." doesn't mean that they will supply one. That is just a generic warning on every spec page.

 

The wording in the c manual is

In general though I suspect the onus would be on Toyota Australia to prove that the warranty-related problem was directly caused by the tow hitch or how it was used. A warranty can't be completely voided without due reason, but this paragraph does give them reason to argue the case if there is a connection.