Don Brash says he and Auckland Council candidate David Rankin were confronted by a man wielding a sword at Mr Rankin's campaign launch in west Auckland on Monday night.
But reporters at the scene have contradicted the Ngapuhi leader's version of events, saying nothing of the sort happened.
"He was in the middle of an interview with TV One News when a car pulled up I think by pure chance, wound the passenger side window down and the guy in the driver's seat started shouting out to David Rankin," Dr Brash told Newshub on Tuesday.
"When David Rankin tried to continue the interview with TV One, the guy parked his car, hopped out with an enormous sword - probably a metre long - and a young boy, who I assumed was his son, and started arguing with David, at which point the interview came rather abruptly to an end."
A crew for Maori news show Te Karere were also present, but both they and the "cowardly" One News team grabbed their gear and left, according to Mr Rankin.
"We were at risk of being seriously assaulted, or worse, and this man's child was in extreme danger, but the reporters just fled the scene."
But TVNZ has flatly denied Mr Rankin's version of events, calling it "incorrect and inflammatory".
In a statement TVNZ head of news and current affairs John Gillespie said the man didn't appear to have any violent intent, and was asking for directions to a local church.
The interviews with Mr Rankin and his supporters were over, and the One News crew was packing up their gear.
"Our team said Mr Rankin approached the man, shook hands with him and spoke to him. Our team were by now in their car ready to leave.
"Dr Brash had left. Mr Rankin got into his car and as he was driving off, rolled down his window and spoke with the team. Our team were the last to leave."
TVNZ says its crew "never felt in danger" and weren't concerned for anyone's safety, at any time. The boy, aged about seven or eight, "was smiling and appeared happy".
The man was apprehended by police in Avondale after a "low-speed pursuit", and has been detained under the care of health authorities. The child was unharmed.
Dr Brash, a former Reserve Bank Governor and ex-leader of the National and ACT parties, says despite the shouting and the sword, he didn't feel threatened.
"It was quite the most unusual experience I think I've ever had… He didn't actually point the sword at us. He just waved it around and it was hard to tell… whether it was a real sword or a ceremonial sword, but he was talking quite incoherently.
"He was either mentally unstable or he was under the influence of some kind of drug."
He believes the man, aged 42 and described as overweight and "not particularly well-dressed", was speaking in Te Reo. The sword had "a rather ornate handle" and "looked as if it could do serious damage if wielded vigorously".
Dr Brash doesn't hold a grudge against the TV crews for scarpering.
"There wasn't much more they could do at that point. They certainly couldn't interview David any longer."
But Mr Rankin wants TVNZ to conduct a "full investigation".
"My main concern was for the young boy," says Dr Brash. "If the man was his father - and we assume that he was - it would have been a quite terrifying experience for him."
Mr Rankin is running as a candidate for the Waitakere ward.