Emergency services, including armed police, were called to the Beach Road Reserve in the suburb of Onerahi about 11am on Thursday.
They found one person who was already dead while the second person who had serious injuries was taken to Northland Base Hospital.
Residents said the incident involved a man and a woman, and that the woman had died.
A homicide investigation has been launched.
Neighbours were upset by the shooting.
Rahera Tau and her family live close to the park and use it often.
She told the New Zealand Herald she felt a bit unsafe.
"It's so sad, especially in a kids' area, by a playground. People won't want to come out here anymore."
Her concerns were echoed online as people said the shooting had left them feeling unsafe.
Whangārei mayor Vince Cocurullo told the New Zealand Herald Onerahi was a lovely suburb.
"Onerahi has always been a very loving, caring community. It's got a great football club and rugby club, and lots of things that the community is involved in.
"This is going to affect the community, there's no doubt about it but the good thing is that there are no safety concerns at all."
Acting Detective Senior Sergeant Shane Pilmer said police did not believe there was any risk to the wider community, but officers wanted to hear from anyone who saw the incident.
"We understand this incident would have been unsettling for the wider community and we appreciate the assistance from the public during this time.
"The key focus for us is determining the series of events that led up to this tragic incident."
