"We got home, started to unpack the car and five minutes later we heard this meow," Mrs McMillan told the Bay of Plenty Times.
They initially thought it was a cat from the neighbourhood but after five minutes realised the noise was coming from their car.
"Initially we were looking under the car - but it was wedged up between the side of the engine and the side of the wheel, where people would lean on or put their rags when doing engine work.
"We were shocked. When my husband heard it, it was coming from the bonnet.
"He panicked a little bit, thinking it can't be in there."
Mrs McMillan said it took her husband about five minutes to get the kitten out.
"She was pretty flustered and hot. She hissed for a little while, obviously it was a bit scared."
The kitten was wedged into the car so badly, it would not have been able to remove itself.
They gave it some water and time to recover - and it was "good as gold".
The next day the couple posted on Facebook about their stowaway kitten and decided they would take it to a vet until its owner could be found.
A post on the Papamoa, New Zealand Facebook page has been shared more than a hundred of times to find the kitten's owner but no one has come forward yet.
Mrs McMillan said the vet believed the kitten was not a stray.
"It's is somebody's pet. It was very, very playful and liked attention.
"They did say it was lucky to be alive. It was pretty hot in there. An engine for four hours, can you imagine how hot it would have been? We called it Lucky, as it was lucky it survived that trip."
Is this your kitten?
* If you think this is your kitten please get in contact with the Forrest Hill Veterinary Clinic & Hospital on 09 410 5169. Proof of ownership will be required to take Lucky home.