Police and SES volunteers carry out a search at Floyd Bay boat launch in Lansvale (Source: AAP Image/Dean Lewins)

A woman has died and a major search continues for two children missing in waters southwest of Sydney.

The trio were seen in distress at Floyd Bay in Lansvale around 10:15 a.m. on Saturday, New South Wales Police said in a statement.

The woman was pulled from the water shortly after help arrived, but could not be resuscitated.

Police said an extensive search was underway for the children, also involving maritime authorities and the state emergency service.

No further details have been released about the people involved or the circumstances leading to the incident.

Anyone with information is asked to contact police.