The attackers made off and the tourists called police from a house close by.
Hawke's Bay police arrested a woman this afternoon over the incident and she has been charged with assault, with intent to rob. Police said further charges are likely.
The arrested woman would appear in the Napier District Court tomorrow.
Detective John McGregor at Napier Police said she was tracked down through the vehicle's registration number, which the tourists had written down.
The tourists were in a "not bad'' state following their ordeal, he said.
"Of course they're shaken, but we're looking after them.''
The gun used to pistol-whip the man was metal, but his injuries were not serious, Mr McGregor said.
The couple were staying in a place that was frequently used by the public and not isolated.
"It's pretty much a random attack and it's hard to say to stay clear of those places...they were just in the wrong place at the wrong time.''
Police are still hunting for the man who was involved in the attack and are appealing for information about him.
The pair were driving a silver Subaru Legacy or similar type of vehicle.
Detective Mike Signal said it was a "a cowardly and despicable act on two innocent tourists who were enjoying a visit to Hawke's Bay".
Mr Signal said the young backpackers were "understandably very shaken by their ordeal".
They had arrived in New Zealand in September for a working holiday, they speak little English and police are using interpreters to help interview them.
Anyone who has information about the attack is asked to contact Mr McGregor at Napier Police on 06 8310 700 or anonymously call Crimestoppers on 0800 555 111.
