The man who was on a mobility scooter was pulled from the tracks by staff at the busy CBD train station after falling from platform 18 about 2.30pm on Wednesday.
He was taken to the Royal Prince Alfred Hospital in Camperdown, where he died, NSW Police said.
There was not a train on the platform or nearby at the time of the incident, and train services were halted, Transport for NSW told 9News.
All lines resumed at 3.23pm but passengers have been warned to expect major delays as police investigate.
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“Passengers can expect flow on delays for T1 North Shore and Western Line, T9 Northern Line, T8 Airport line, T2 Inner West & Leppington line and T3 Bankstown line,” Sydney Trains said.
“We remind commuters to be vigilant while on platforms.”