Mrs Hartell said she turned around and tried to thank him, but he had walked away.
The man looked to be in his 50s she said. She said the staff member told her she had seen him do it before.
She posted her story on Facebook to thank him.
Online she posted, "I have had a stressful two weeks. Last week I nearly lost my father and had to find money to get down to Wellington Hospital, and this week my mother in law is very ill in hospital.
" So this guy was an angel who not only helped me out, he made my day."
The Facebook post has attracted "likes" from all over the world including England, America and Australia.
Foodstuffs, which owns Pak'nSave said it had been in touch with the man, who did not want to be identified.
"He just likes the idea of doing something good, and it possibly coming back to him one day," spokeswoman Antoinette Shallue said.
He wanted people to know there were still Good Samaritans.
He said it was the first time he had come to someone else's aid - contrary to the supermarket worker's comment - but it probably would not be the last.
Ms Shallue said it was not the first time she had heard of this happening.
People quite often helped out in the stores and goodwill could make a huge difference to someone's day.
"It doesn't happen every day, but I think Kiwis are pretty generous and if they see someone in strife they'll help them out."
- APN, APNZ