A gun-toting thug picked the wrong couple to pull a pistol on at the Hamilton Gardens yesterday.

Instead of pocketing one or two wallets from frightened tourists, the 19-year-old found himself disarmed and pinned against the tarseal.

The attempted mugging is the latest in a string of incidents in or near the city's prime tourist attraction, casting further doubt on the area's safety.

Chartwell man Ian Morgan, 69, witnessed the incident and stepped in to help.

He runs a sober driving service and was waiting to pick up clients near the gate one barrier about 1am when he saw a couple walk past.

They had been at a charity auction to help send police dog handler and armed offenders squad member Blair Spalding to Germany for specialist neurosurgery to remove a brain tumour.

Morgan noticed another figure on the couple's tail.

The situation escalated at the road barrier when the balaclava-clad man pulled out a gun, Morgan said.

"Some discussion went on and next minute I see the man push the gun away and slam the guy on the ground. The guy made a bit of a mistake because the man he confronted was an off-duty policeman.

"The policeman had him on the ground, and I went along and helped to restrain him. When we realised there was a gun involved, we weren't that keen on letting him up again."

The offender remained pinned to the road until police arrived and arrested him.

The officer is based in Hamilton but is now on holiday.

Morgan was "amazed" and full of praise for the officer's actions.

"I can't speak highly enough of it.

"It's a pretty brave thing to confront a guy who has a gun on him."

Morgan suspected the pistol was a replica but said that when it was pointed at you in the early morning darkness "it's real".

"There's a quote that says the only thing required for evil to succeed is for good men to do nothing - so you can't just sit back.

"If it had been somebody who didn't stand up for themselves, it may well have been a totally different scenario."

It's the latest in a string of similar incidents in the area.

Last Monday, a 60-year-old Chinese tourist was attacked at knifepoint on the edge of the Hamilton Gardens.

The knife-wielding man cut the woman's shirt before severing her money belt and fleeing along the riverbank walkway with an accomplice.

The daylight attack followed another incident near the Waikato River the previous Monday when a woman was approached by a man demanding money but he ran off when she refused his demands.

Hamilton Mayor Julie Hardaker said she had discussed the issue of safety on the city's river walks with police, who were taking action to address it.

On a more positive note, there was a success story linked to the would-be-mugger's arrest with Spalding's charity auction raising $70,000 towards the $150,000 needed for his surgery.