A five-year-old boy and his stepmum have been taken to hospital after they were rescued from freezing water off Phillip Island.

Ambulance Victoria says the little boy slipped and fell off a jetty into the water near Cowes.

His 44-year-old stepmum jumped in and tried to save him.

It is believed a teenage boy who saw the pair struggling in the water came to the rescue.

The woman and child suffered severe hypothermia and are in serious but stable conditions.

The child was taken to the Royal Children's Hospital, while the woman was flown to the Royal Melbourne Hospital.

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