Tim Shaddock set out for French Polynesia from Mexico in April but his catamaran was badly damaged by a storm just a month into the journey.

The 51-year-old and his dog Bella survived under a small canopy on board the battered vessel for weeks before they were spotted on Thursday by a helicopter that was on the lookout for tuna for a fishing vessel.

A doctor who is monitoring Mr Shaddock's health from the shore and working with the trawler's captain told Australia's 9News the sailor has "normal vital signs" and is "stable and very well".

In a video, Mr Shaddock says: "I've been through a very difficult ordeal at sea and I'm just needing rest and good food because I've been alone at sea a long time. I've not had enough food for a long time."

Mr Shaddock, who is from Sydney, was left drifting in the north Pacific Ocean after being hit by bad weather weeks after he set sail from La Paz in Mexico's Baja California peninsula.

The electronics on his vessel were wiped out by the storm leaving him unable to call for help.

He survived by collecting rainwater and eating raw fish.

Ocean survival expert Professor Mike Tipton, from the University of Portsmouth, told Sky News Mr Shaddock's survival was a combination of luck and skill.

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"He was in a warm environment so didn't need to worry about hypothermia," Prof Tipton said, having earlier compared Mr Shaddock's survival ordeal to the Tom Hanks movie Cast Away.

"Nearly all the long survival voyages occur in warm water," the professor added. "He had a supply of fresh rainwater. He did the correct thing of minimising activity in the hottest part of the day to reduce sweating."

Prof Tipton said at least 110ml of fluid per day is the minimum needed for survival and it is vital not to use too much energy to decrease food requirements.

An image of Mr Shaddock soon after his rescue showed him smiling and wearing a blood pressure monitor on his arm.

It's understood the tuna trawler is now heading to Mexico where Mr Shaddock will undergo medical treatment.