Police and SES volunteers conduct a search at Floyd Bay boat ramp in Lansvale [Source: AAP Image/Dean Lewins]

A woman has died and a major search is continuing for two children missing in waters in Sydney’s southwest.

The trio were seen in distress in Floyd Bay in Lansvale at about 10.15am on Saturday, NSW police said in a statement.

The woman was pulled from the water shortly after emergency services arrived but could not be revived.

Police said an extensive search was underway for the children also involving marine authorities and the State Emergency Service.

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

Anyone with information has been urged to contact police.