A submarine has been spotted cruising through Port Phillip Bay, much to the surprise of Melburnians who were working or out for a stroll in Williamstown on Monday.

A 3AW listener managed to snap a picture of the vessel as it sailed towards the West Gate Bridge.

The submarine seen heading towards the West Gate Bridge. Credit:Grant O'Connor / Facebook

The submarine has generated a buzz online, with one Reddit user joking the ship's occupants must be docking just in time to watch Tuesday's Melbourne Cup.