Mr Gu thought that was the end of it but on Thursday morning the man's employer delivered a handwritten apology and $100 cash for the donation box. It was thought $40-$50 was in the box when it was stolen.
In the letter the man gave his name and expressed his remorse for his "despicable act".
Mr Gu said he was pleased the thief had owned up and was convinced the apology was sincere. He had forgiven him and urged others to do the same.
The man's boss was giving him a chance to make a fresh start in life, Mr Gu said.
The outcome, and the positive reaction to the apology on a local Facebook page, made him glad to live in Northland.
"We want him to see that people welcome his actions. It will encourage him to do better."
The sushi shop has a new donation box which is now chained to the counter.
Kidney Kids NZ chief executive Keith Mackenzie said he was thankful for the apology and the donation.
"We hope he has learnt from his mistake ... these donations are very important to the children we support."
Last month an Opua business owner also gave $50 to Kidney Kids to make up for the stolen donation box.