A man who former National Party leader Don Brash says wielded a metre-long sword at him has been detained by health authorities, police say.

Dr Brash said he was out with local body political candidate David Rankin in west Auckland last night, when they were approached by an agitated man, who had a young boy with him.

Photo: ACT Party

A television crew was with Dr Brash and Mr Rankin at the time.

"I didn't quite understand what was going on, but eventually he got out of the car and with his young son, who would've been eight or 10 I guess, started waving this very large sword around," Dr Brash said.

"It was quite the most unusual experience I've ever seen."

The man was speaking "quite irrationally", Dr Brash said.

"It was hard to tell whether he was under the influence of something or whether he was literally mentally unstable."

Dr Brash did not call the police, he said.

"He didn't look like he was going to attack someone with the sword, he was just waving it around."

Mr Rankin gave a more dramatic account of the incident, saying in a press statement that the man "was ranting hysterically and threatening us with his sword".

"My main concern was for that young boy," Mr Rankin said.

"His father was a real threat to his safety."

Police said officers spotted the man's car in Kelston and followed it in a "low-speed pursuit".

"Police managed to get alongside the car and officers engaged the driver in conversation, appealing for him to stop and reminding him of the child in the car," Inspector Scott Webb said.

Officer eventually managed to block the car in an Avondale street.

"The 42 year-old driver was assessed and has been detained for treatment under the care of health authorities," Mr Webb said.

The child in the car was unharmed and police had been in contact with other government agencies to make sure he had ongoing care.

Mr Rankin had laid a complaint with TVNZ, whose camera crew left during the incident, he said.

One of his supporters had called police and given a detailed description of the man and his vehicle, Mr Rankin said.