If complaints are received, the NZTA weighs the potential for a plate to offend against the plate-holder's freedom of expression under the Bill of Rights.
"Making this determination can be very difficult and the NZTA considers all such complaints carefully," said customer access manager Sue Hardiman.
NZTA central region media manager Anthony Frith said "offence" could be subjective.
"There can be subjective elements to interpretations of the guidelines. Where some people would find (a plate) offensive, others would think we were being overly harsh."
Mr Frith said the applications that were turned down did not reflect badly on Personalised Plates Ltd, but it was good that NZTA could step in as a "safety net".
The agency has already banned some three-letter combinations, including plates beginning with FAK, FAG and FKN.
Plates containing the letters I, O and V were usually declined because they could easily be mistaken for 1, 0 and U, which caused problems for police.
If an existing plate is found to violate guidelines, NZTA has the power to demand its surrender. It did this once in the past year, for PIG5HT.
Police can also apply to NZTA to have plates cancelled.
Bad-taste plates are still popular, though, and some end up on Trade Me. The auction site has the plate B1TSH for sale for $3,000 and IM A H8R for $999.
Trade Me spokesman Paul Ford said the popularity of personalised plates on the site was rising.
