Two inflatable rescue boats and two jet skis were launched, joining a rescue effort including the Police Eagle, the Northern Rescue Helicopter and Coastguard NZ.
Surf lifeguards retrieved a 5-year-old child from the water, leaving one male still missing, Surf Life Saving said.
Auckland Westpac Rescue Helicopter, the Eagle helicopter and Surf Life Saving performed an extensive grid search for him.
The search continued until about 8pm when it was considered too dark to continue.
A rāhui has been placed over Port Waikato and surrounding areas until 8pm tomorrow by the people of Te Pūaha o Waikato.
It affects Sunset Beach and the Waikato River from Port Waikato to Kariotahi Beach.
Coastguard NZ said: “Our thoughts are with the family and friends of the missing individual. We will continue to support police in the search over the coming days.
“A special acknowledgment to all our volunteers and staff who responded to this incident, as well as others across the motu this Christmas. Thank you for your sacrifice and dedication to helping others on the water.”
Auckland Coastguard Air Patrol volunteers continued to search for the missing person today.