Police are still searching for the alleged flasher who victims say exposed himself on city-bound trains on the Sandringham and Werribee lines on New Year's Day.

Investigators have been told the man exposed himself on three separate occasions between 8.30am and midday.

He allegedly exposed himself twice while travelling on the Sandringham line from Hampton to Southern Cross Station.

It is believed he then boarded a Werribee-bound train at Southern Cross Station and got off at Newport.

From there, he took another train back towards the CBD and indecently exposed himself between Newport and Yarraville.

He was last seen getting off the train at Yarraville.

Police have released images taken from CCTV footage of a man they wish to speak to.

The man is believed to be aged in his mid to late 20s and is described as being about 178 centimetres tall, with a medium build and auburn coloured hair.

He appeared to have several days' growth of facial hair at the time and was wearing a light green, long-sleeved grandpa style top and dark blue jeans.

Anyone with information is asked to contact Crime Stoppers on 1800 333 000.