The woman claims the staff member behind the counter kept talking about the sushi.
"We placed our order, but then in every gap in our conversation she would really insist that what my son was doing was not allowed and he shouldn't be doing that and we were going through the same thing, over and over again ..."
Annoyed, they cancelled their order and walked out.
Avin Jit, whose parents, Bishwa and Geeta Jit, have owned the restaurant for eight years, said his mother felt bad but they had been trying to be more strict about their policy as they had just spent thousands on renovating their restaurant.
"It was quite busy and we just can't allow that stuff, because all of a sudden at busy times we'll start seeing McDonald's wrappers or Burger King wrappers, and we'll be like, 'What's going on here? We're Nando's'," Mr Jit said.
"I know where they're coming from. If it was my child I might be annoyed too, but it's a policy."
Mr Jit said that earlier on the night, staff had to tell a regular customer not to bring in McDonald's.
Hospitality Association chief executive Bruce Robertson said most businesses would not expect people to bring in food unless they had certain dietary requirements or the age of their child was an issue.
"It's the exception rather than the rule and, to be fair, most consumers are reasonable about that and most of them will have a reason for it, like dietary requirements or things like that."
Andy Lucas, director and marketing manager for The Coffee Club New Zealand, said they "occasionally" experienced people bringing food into their cafes and restaurants, "especially if they are with younger children who might be teething or going through a fussy phase".
"However, as this isn't a hugely regular occurrence, we choose to turn a blind eye. We certainly wouldn't be asking any of our customers to throw their food away."