Police allege the ammunition, which was inside several ice cream containers, was located in the boot of the car.
The man did not have a firearms licence and could not provide a good reason as to why the ammunition was in the car, Mr Sutton said.
Although the man lived rurally, he was not believed to be a hunter.
It was unusual for police to come across such a large amount of ammunition, Mr Sutton said.
"[It's] not that often - enough to ring alarm bells when you do."
A knife was also found in the car, he said.
It was believed the man may have been sleeping in his car before being arrested.
The officer in charge of the case, Constable Selena Edlington, said the collection of ammunition was found among other rubbish in the car.
"It may not necessarily have come from one place."
A Masterton man has since been charged with driving while disqualified and unlawful possession of ammunition.
Police were urging anyone who might recognise the ammunition and the distinctive containers it was stored in, to come forward.
Mr Sutton said: "We are not too sure where it has all come from."
Anyone wishing to claim the ammunition would need to provide sufficient details to police.
