A young diver has 'booped' the nose of a southern right whale off the coast of Tasmania, but warned others "harassing whales could be the last thing you'll ever do".

Kaeo Landon-Lane, 19, was with his dad Chris south of the Douglas River mouth, near Bicheno on Tasmania's east coast, on Sunday when he chanced upon two whales near the surface.

"We stopped our small 6 horsepower motor (boat) over 100 metres away and I hopped in the water. I swam away from the boat and they shadowed into view a couple of times, where I would stay completely still. The visibility was at least 20m," Mr Landon-Lane said.

"The third time they came into view they were pointing at me, and slowly coming closer. There were a lot of things going through my head, but fear was not high on that list. They didn't feel threatening. It got so close I could see the hairs on her chin.

"I remember thinking, 'that's close enough now'.

"It aimed for the centre of my chest. I didn't feel comfortable with the chest bump, so I reached out my hand so that I could keep some small distance."

Mr Landon-Lane said pushing against the whale, "I felt its enormity, being that close to it and feeling so small".

"It held that touch for a moment, before backing up with its tail and moving past me. My thigh was now on another collision course with its pectoral flipper, which gently bumped me sideways as it glided past.

"Biggest nose boop ever.

Mr Landon-Lane said his father was apprehensive as the whale approached.

"He called out and asked if I wanted him to pull me out," he said.

Authorities warn against coming too close to whales in the wild. ( Supplied: Storyful )

"They returned once I swam back to the boat ... floating past slowly three more times. My dad then got in the water as well, and they came back another four times, kicking with their (tails) in slow motion."

The encounter was filmed from the boat by Mr Landon-Lane's dad, Chris.

He described the whales as "gentle, gentle giants", but cautioned others of getting too close.

"I want to make it clear that humans touching whales is not OK, and that this whale initiated this contact. Harassing a whale could be the last thing you do. Whales are curious and intelligent creatures, if you're interesting then they'll hang around," Mr Landon-Lane said.

The Tasmanian Parks and Wildlife Department advises the southern right whale species is one of three whales listed as endangered under Tasmania's Threatened Species Protection Act.

"Although whales may commonly appear placid at sea, and dolphins often initiate contact with humans directly, it is important not to disturb cetaceans by approaching closely," it said.

The department recommends a "minimum approach distance of 100m when encountering any cetacean species".

STORYFUL/ABC