"I ran and told Mummy," she said.
Mary had been playing on her swing with her younger sister, Ainsley, when she saw the smoke coming from the Berwick Rd area, near Waihola, late last month.
She told her mother, Robyn, to call the fire service and rushed to tell her father.
Her father, Richard, a volunteer with the Waihola Voluntary Rural Fire Force, and the fire chief went to check the fire, a permitted burn-off which was getting out of control in the hot, dry conditions.
"It could have been a catastrophe if it entered the Berwick Forest," Mr Hay said.
A man who had been tending to the vegetation burnoff said he was about to call the fire service.
Taking two and a-half hours to control, the fire could have been worse but for the observant 4-year-old, who was sent an official letter of appreciation.
"It was through your quick actions that the fire brigade was able to respond quickly to your call, therefore stopping a serious fire becoming a danger to life and property," the letter said.
"I am very proud of her," Mr Hay said.
