The whale was likely heading south as part of its migration from sub-tropical areas to sub-Antarctic feeding zones, and had carried the rope for a significant distance given its level of emaciation.

Marine biologists made the findings during a necropsy at the beach on Wednesday.

Sei whale strandings were described as "rare" in Tasmania given the creatures are normally elusive, so the whale's skull is likely to be obtained by the Tasmanian Museum once the decomposition process finishes in a few years.

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Marine Conservation Program wildlife biologist Kris Carlyon said the rope was tangled through the baleen plates, which help the whale filter prey from the water.

"What we think is likely to have happened is that when the whale sucks in a huge amount of water and filters out the prey, the rope caused too big a gap so the prey was able to slip through," he said.

"We have no idea where the rope was from. It could've been picked up anywhere between the sub-tropical and sub-Antarctic areas.

"From what it looks like, the whale has been able to feed a little bit so it's obviously been carrying the rope for a significant amount of time."

Dr Carlyon said sei whales are "quite cryptic" and little is known about their movements, although they usually travel either solo or in pods of up to five.

Samples were taken from the whale on Wednesday to aid in global research into the species. The remainder of the whale was buried.

"Whilst strandings are extremely sad, they are an opportunity to learn more about these species that we rarely get to see," Dr Carlyon said.

"When we talk about rare, the species itself isn't rare, but having it strand on the beach is rare."

As ocean debris continues to mount, entanglement has been recognised as a threat worldwide for all whale and dolphin species.

While some whale species such as the humpback have rapidly returned to pre-hunting levels, other species like the southern right whale were not increasing as fast in number.

The sei whale stranding came shortly after the unrelated stranding of a pygmy right whale on another beach near Bridport.

Dr Carlyon said they were unable to determine a cause of death for the pygmy right whale.

"It clearly had some underlying issue as it was emaciated, but we couldn't determine a cause," he said.

"There is no indication that the events are linked in any way.

"Because pygmy right whales are more likely to be resident around the Tasmanian coastline, rather than migratory, it's not a surprise to see the odd individual turn up dead on the shores."