Police are searching for a missing woman, after she failed to return from a swim off Haumoana in Hawke's Bay.
Reports say two children were in the water on Tuesday afternoon together, when one began to have difficulty swimming.
"At this stage, we know that a young female has entered the water, got into difficulty and we believe that it's her mother who has gone in to rescue her," Senior Sergeant Martin James told Newshub.
The woman went into the lagoon to rescue the child in difficulty, but while the children made it back to shore, she did not.
Police were alerted at 4:50pm and the children were taken to hospital in a serious condition.
Coastguard, surf life savers and a helicopter joined the search, which continued until nightfall and is expected to resume on Wednesday morning.
A witness to the incident described sirens arriving suddenly and people running across the beach to direct the search.
George Araia told Newshub he saw the day's events unfold and both children looked "pretty sick" when they were brought to shore.
Earlier in the day, a man drowned at Lake Rotorua, after he was pulled from the water and CPR attempts to revive him failed.