"They cut her off with their car. They have pulled in front of her car then one of the young offenders has hopped out. They're all dressed in the same black caps and clothing and wearing those black satchel bags that you wear over the shoulder."
The young man then told her he knew she had money and to hand it over.
"She gave him a look like, 'are you actually doing this to me?' because of course she was a bit taken aback by it and that's when he reached into his satchel and pulled out what we believe was a sawn-off shotgun."
The car, a silver Mitsubishi Legnum, then took off. However, the robber took off on foot, through the backyard of properties jumping over fences.
He said his stepdaughter had handled the robbery but it had left her shaken.
"She's just working through it. It's changed her perception of a lot of things. She's only 20."
She was now considering selling her car.
His post has now been shared by friends and others on Facebook more than 800 times.
They had been interviewed by police who were looking at CCTV footage from the shopping centre, Williams said.
"It's a bit scary. I put a post on social media for people to be aware because we can all go about our daily lives and get a bit of tunnel vision and we just need to look out for each other."
Waikato police said they were investigating.