Passengers on board a Port Douglas reef tour were greeted with the rare sight of a Whale Shark at Agincourt Reef today.

Silversonic passengers had the opportunity to see and swim with the magnificent marine creature measuring four to five metres in length.

Silversonic’s Skipper, Cliff Stanaway said the whale shark swam along the southern end of Agincourt three ribbon reef and a number of guests were snorkeling and diving at the time.

“We first spotted the whale shark as it swam past a few metres from the boat and created a shadow over the top of the intro divers. This was certainly a special day for our passengers and crew,” he said. 

Quicksilver Dive Instructor Kylie McQuie who had two intro divers underwater said it was an amazing experience.

“Not being able to talk underwater and to see this majestic animal swim past, it left everyone speechless literally. This was a huge motivator for our guests to become certified divers,” she said.

The whale shark may be the world's largest fish reaching lengths of 12 meters or more but, according to marine scientists, locating them on the Great Barrier Reef is like "finding a needle in a haystack".  

Unlike Western Australia's Ningaloo Marine Park, a known aggregation site the species frequents, sightings off the east coast are sporadic.

James Cook University's Adam Barnett said the habitat use and movements of whale sharks within the Great Barrier Reef Marine Park was one of the great mysteries of the deep.

"We have no idea how important the Great Barrier Reef is to them, we know nothing, so this research is just the tip of the iceberg," Dr Barnett said.

VIDEO | See video of the animal on the reef today below: Indepth Video, Kai Cheng.