The road was shut until the ammunition could be reloaded onto another truck and removed, he said.
The crash happened 6km south of Waitahanui when the trailer overturned on a mild bend. Nobody was injured.
Senior Sergeant Karl Konlechner of the Bay of Plenty Police District Command Centre said one of the truck tyres had blown out and that was what appeared to have caused the crash.
"They closed the highway as a precaution, but the ammunition was quite stable.
"There will be a full investigation of the scene and an interview with the driver to fully determine the cause of the crash."
Mr Konlechner said it was too early to say whether charges would be laid.