Personalised number plates bearing the flight number of the Malaysia Airlines ghost flight have been sold since the jetliner vanished on March 8.

Personalised Plates owner Jeremy Lubeck said the plates MH370 and MH37O — the latter in bright red markings — had been sold by his company since the flight disappeared. Each was sold for $599.

The red-coloured plate has been spotted on a Holden vehicle in Auckland, but the other plate remains on layby and has not yet been sent to its owners.

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Transport Agency spokesman Andrew Knackstedt was not aware of the plates and, while there were restrictions on what number plates could be registered, the motivation of personalised plate buyers was not always clear.

"You have to see how it will be be viewed by society at large. It is subjective, but we still need to draw that line somewhere."

Plates were not issued if they promoted violence, discrimination, bias, illegal substances or activities, or encouraged unsafe driving. They must also not be derogatory to the police or public figures, breach intellectual property rights or contain bad language.

Mr Plates director Bruno Szajer runs a website that trades thousands of number plates in New Zealand and Australia.

Szajer said he had not heard of the MH370-related plates, and thought they were "a bit shocking".

The sale of plates related to world events — both joyful and tragic — was common, he said.

However, the most common personalised plate purchases remained those related to new cars, especially expensive models.