A Sydney university student who dived into wild seas at Bondi a day ago is still missing and feared to be the fourth victim of the state's destructive weather system.

Twenty-year-old Endicott Ackerman was seen jumping into the ocean, just south of Icebergs at Bondi, on Monday afternoon but never resurfaced.

The University of Sydney student was dressed only in his board shorts when he jumped off the rocks and into waves that towered up to several metres as a result of the weekend's powerful east coast low.