It seems like Phillip Island is becoming quite the super-highway for submarines.

An eagle-eyed reader spotted this submarine off the Nobbies and Kitty Miller Bay this morning.

Photographer Gary Parnell also snapped a shot of the sub off Cowes.

In March, residents of Silverleaves saw a submarine surface just off Cowes, around 8am one morning.

At the time, the Defence Department confirmed that a Royal Australian Navy submarine was "operating in the Eastern Australian waters", but did not provide any explanation or details about the submarine's appearance on our shores.

"Navy's submarines regularly train in Australian waters and visit Australian ports," a Defence spokesperson told the Advertiser in March

"For operational security reasons, Defence does not provide the names and positions of their submarines whilst at sea."