Pieces of plastic found in a dead whale's stomach have prompted authorities to repeat a warning against marine litter.

Government marine officer Isabel Beasley on Thursday said a post-mortem examination on an adult pygmy right whale washed up on Tasmania's east coast found two plastic caps in its stomach.

The plastic tops were about four centimetres in diameter, reasonably large to be carrying around in the stomach of the six-metre long, baleen whale, Dr Beasley said.

"Although we don't know if the plastic tops caused the death of the animal, we do know that sickness or debilitation can cause whales to come inshore and strand," she said.

Dr Beasley said the discovery of the plastic tops is a reminder as to why people need to be careful with the way rubbish and marine debris is disposed of around our waterways.

"It's not just being aware of disposal of your own rubbish but also removing rubbish you may see in the water when you are out there," she said.

Although pygmy right whales have been recorded around Tasmanian waters previously, they are the least known of all the baleen whales.

While rarely seen at sea generally, the whales are found in temperate and sub-Antarctic waters, but as they are usually found further offshore it makes studying them much more difficult.

Dr Beasley said scientists took biological samples from this whale to increase knowledge of the species.

AAP