Police said the woman got into trouble on Baylys Beach, on the west coast near Dargaville, around 5.45pm yesterday.
Police said the woman went to the aid of her child who was struggling in the water. Her child received minor injuries.
In a statement police said: ''This is an absolute tragedy and Police's deepest sympathies are with the woman's family.''
The matter will be referred to the Coroner.
The Tasman Sea batters Baylys Beach and the surf is notoriously dangerous. A number of people have died on the beach.
Dylan Robinson, 13, of Henderson, West Auckland, drowned in August 2012, on the beach and two people drowned there in one week in January and February 2006 - 46-year-old Alexander Stewart and Kevan Moore.
The beach is patrolled over parts of the summer by lifeguards and lifeguards there are waiting for new facilities to be built.
