A massive oil tanker is believed to have run aground in Port Phillip Bay, off Rosebud.

The 115-metre Bow Singapore hit a sandbar while turning in a shipping channel, just west of the Hovell Pile beacon, it is believed.

The tanker which can carry up to 6219 tonnes of oil or chemical products was inbound, heading through a rain squall to Geelong when the accident happened.

A file image of the Bow Singapore. Credit:Walter Pless / Marinetraffic.com

Blairgowrie sailor Chris Hallam was watching the boats on the water before sunset when he noticed the ship was stranded about 6pm on Friday.