Gold Coast theme park Sea World says it is "deeply saddened" by the death of one of its polar bears over the weekend.

Liya, 19, was born in Russia and was moved to Sea World in 2001 as part of an international breeding program.

She died at the theme park on Saturday night.

The cause of death is not yet known however the park's head of marine sciences Wayne Phillips said Liya's death was unexpected.

"She was an amazing bear and a wonderful ambassador for her species and for climate change," he said.

"Liya was normal in behaviour and demeanour leading up to Saturday evening, but at the same time, Liya — being 19 —had reached the normal life expectancy of a polar bear."

Polar bear Liya gave birth to two cubs during her time at Sea World. ( Supplied: Sea World )

'The first polar bear to give birth in Australia'

Liya had two cubs, Mishka, who also lives at Sea World, and Henry, who was sent to Canada several years ago.

Mr Phillips said Mishka was coping fine, having been weaned years ago.

He said the polar bear would leave an enduring legacy at the theme park and around the world with all the visitors that saw her.

"We're proud that Liya lived a full and enriched life here at Sea World and that her public awareness that she gave to millions and millions of visitors over the years is a legacy that we'd like to continue," he said.

He said the theme park had three other polar bears and had not thought about replacing Liya yet.

"We'll talk to the international breeding groups and see if they would like to continue breeding and in that case we may need to look at the genetics of our group and decide whether we bring in another bear," he said.

Polar bears live about 15 to 18 years in the wild.

Sea World said the health and wellbeing of animals in its care was its highest priority.