Emergency services received multiple reports of storm damage across the area from 3.30pm on Sunday, with trees and powerlines down across suburbs from Mona Vale to Forestville.
Police, New South Wales ambulance paramedics, Fire and Rescue NSW, and rescue helicopters responded to the incident at a car park on Ocean Street, Narrabeen, after reports a tree and powerlines had come down.
A woman believed to be aged in her 70s died at the scene.
NSW ambulance said a woman in her late teens was treated for spinal and head injuries, while another woman aged in her 70s was treated for multiple injuries. Both were taken to Royal North Shore hospital in a serious condition.
A young boy was also assessed for minor injuries but did not need to go to hospital.
Elsewhere in NSW, severe thunderstorms are likely to produce damaging winds over the next several hours in parts of the Central Tablelands, Southern Tablelands, Central West Slopes and Plains and South West Slopes districts.
Meanwhile in Victoria, a severe thunderstorm warning for damaging winds and heavy rainfall has been issued for those in East Gippsland, West and South Gippsland and parts of North Central and North East forecast districts.
