Rebecca Oldham had already been put under anesthesia in November when surgeons discovered she was 32 weeks pregnant with a 9-pound baby.
"I was facing not being able to have any more children because they thought there were problems with my ovaries and all of a sudden we had a son," she said, adding that she and husband James Tipene have a 20-month-old daughter, Hayley.
Doctors woke Oldham up from anesthesia to tell her they would perform a caesarian section to deliver her baby instead of remove her ovaries.
"I am so glad they woke me and told me I was going to have another baby," she said. "Even though it was short notice it was better than waking up and being handed a baby."
Oldham told the New Zealand Herald she had had blood tests and other tests after she started complaining of pain in her abdomen, but doctors never discovered her pregnancy.
Oldham and Tipene named the surprise baby James.
One Serbian study suggests up to one in 7,225 women don't realize they're pregnant until delivery.