A group of reef goers watched in amazement as they encountered an awe-inspiring whale shark after a day of diving on Tuesday.

Travelling through the shipping channel near Tongue Reef the group spotted a dark shadow, estimated to be around 15 feet long, and stopped to investigate.

"We have lived in Port Douglas for 28 years and have had boats all of this time  We have never seen a whale shark before despite spending a lot of time on the reefs off Port Douglas," said Lillian who was one of the lucky spectators.    

"The whale shark was travelling along close to the top of the water...He was not disturbed at all.  He just kept swimming along slowly close to the surface.  We were all ecstatic (and) could not believe what we had experienced."

The five-minute experience is sure to leave a lifetime of memories.

Tessa Bickford of Great Barrier Reef tour company Poseidon Sailaway said the group was extremely lucky to have come across the whale shark.

"The most common place to find them in Australia is off the coast off Western Australia.

"Minke whales and humpback whales are a pretty common occurrence for us up here in Far North Queensland, especially during the winter months, so to see a whale shark here in summer is a pretty rare experience. It is a very lucky encounter for them."