A man who took a bullet for his country is getting over the ordeal of being "interrogated" after he tried to get into a gun buyback event with his support dog in Canterbury.

Dion Taka took his service dog, Delta, with him when he went to hand his guns over in Rakaia on Saturday. It was already a daunting prospect for the war veteran.

Badly wounded when he was shot in Afghanistan in 2012 in the Battle of Baghak, he now lives with post-traumatic stress disorder (PTSD) and type-2 complex regional pain syndrome.

For the past two-and-a-half years, Taka has been able to resume some of his independence with Delta by his side. She is able to recognise his anxiety and calm him down, and performs a raft of other duties including fetching medication.

"It took me a couple of weeks to decide to go [to the gun buyback], but I knew I had to," Taka said. "I just put on a happy, brave front to get though it."

He walked in and was greeted by a row of police officers down one side and a row of contractors down the other, all dressed in black.

Taka said one of the contractors immediately called him back to put his rifle and accessories on the desk, and tie Delta, who was wearing her service jacket, up outside.

"I told him, 'she is a service dog, she is working'. But he told me again to tie her up outside."

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When Taka again refused, the contractor asked why he needed a service dog.

"I told him, 'for situations like this. I have PTSD'. Then he asked me what I was there for, which was a pretty stupid question. I told him was there to hand my guns in."

Taka said the contractor then whistled at him "like a dog" two or three times and asked if he ought to own a gun if he had PTSD.

He said the contractor also told him he should "see someone" instead of just having a dog to stroke.

He said the experience left him feeling like he had been "interrogated", while his wife, Frances Taka, said she had "had enough".

"People in this country need to treat other people properly," she said.

"He took a bullet for his country and he was doing the right thing by taking his guns back. I am just disgusted."

Inspector Craig McKay, who was at the Rakaia event and spoke with Dion Taka, said police took the safety and security of those attending amnesty and buyback events very seriously.

"Police acknowledge that the man was initially told he could not take his dog into the event, however, after an amicable conversation with a senior officer, the man was able to proceed with his dog and his firearms were processed without any further issue," he said.

"Police apologised to the man for the misunderstanding but are confident he was spoken to in a respectful and professional manner."

There had been a positive response from the nearly 20,000 people who had handed in firearms to date, McKay said.

Kotuku Foundation Assistance Animals Aotearoa (KFAAA) founder Merenia Donne said Delta was the only working dog in New Zealand specifically trained in PTSD.

Taka's experience was appalling and contravened the Human Rights Act, she said. People had no right to question people's disability when they were with a service dog.

"The bulk of the people we help have an invisible disability," she said.

"There is a huge stigma around mental health conditions and for individuals to make cheap comments like they did [on Saturday] is inexcusable."

Donne founded KFAAA in 2006 and trains dogs like Delta for specific purposes. The group does not have a puppy breeding programme and often trains up a family's existing dog.