Fairfax said tribunal referee Clayton Luke said the most - but still reasonable - a key would cost was about $200.
As a result, Toyota will now have to pay the $325.62 difference.
Luke said Toyota's claim had been misleading and that it did not disclose a "significant portion'' of the price charged by the dealership, Armstrongs Auckland.
Toyota breached the Consumer Guarantees Act that outlines that a manufacturer must guarantee reasonable availability of spare parts.
"I am satisfied that the price charged by Armstrongs was not a price whereby Toyota New Zealand could say the keys as spare parts are reasonably available,'' Luke said.
"The part is simply not reasonably available when what is being charged is more than 10 times its manufacturing cost and that cost is likely to grow to some 10 per cent to 20 per cent of the value of the vehicle within the reasonable lifetime of the vehicle.''