Key Points:

A nine-year-old boy has survived a family tragedy which killed his mother and five-year-old sister on mudflats in south Auckland.

The boy was spotted by a searching police helicopter with his mother and sister's bodies about 9am today, several hours after an initial alarm was raised.

Police this evening named the mother and daughter as Tangatavai Tania Mataora, 43, and five-year-old Maeva Precious Mataora.

Police said the family of four - a mother and father and their two children - went floundering about midnight on mudflats on the southern shores of Waitemata Harbour.

Inspector Gary Hill said the father of the family became separated from his wife and two children.

"The father heard his family in distress but could not see or find them due to the darkness.

"The father has walked to a nearby farmhouse and raised the alarm at approximately 4am this morning," Mr Hill said.

The Westpac rescue helicopter and a police search and rescue team began searching for the missing family members but could not locate them, he said.

He said about 9am today the police Eagle helicopter found the family members on mudflats about four kilometres from where they had gone into the area.

When police search and rescue members reached the family members they discovered Ms Mataora and her young daughter were deceased.

"The nine-year-old son was found nearby safe and well," Mr Hill said.