Mainland Australia is bracing for a heatwave so intense it could rewrite the history books, but it is not just the land that is sweltering.

Key points: A heatwave that caused successive 40-degree days in WA is now heading east

A heatwave that caused successive 40-degree days in WA is now heading east The heat has caused sea temperatures to rise markedly off the state's coast

The heat has caused sea temperatures to rise markedly off the state's coast Fish kills attributed to the heat have been recorded from the Pilbara to the South West

Western Australia's coastline is in the midst of the most widespread marine heatwave it has experienced since reliable satellite monitoring began in 1993.

The warm waters are believed to have contributed to a number of fish kills in the past month.

"Particularly in the last two weeks or so the ocean temperatures have been increasing — they're about two degrees warmer than what is normal for December," University of Western Australia coastal oceanography Professor Charitha Pattiaratchi said.

"It's unusual weather because we have summer conditions that are usually experienced in February occurring in December."

A marine heatwave is defined as five or more days when sea surface temperatures are warmer than 90 per cent of the previous observations at the same time of year.

The waters off WA met that threshold this week.

WA's coastline has seen temperatures two degrees warmer than what has previously been observed in December. ( Supplied: BOM )

Fish kills linked to changing environment

A combination of high temperatures and low tide is believed to have contributed to the deaths of tiny crabs in Karratha mudflats, wild oysters at the mouth of the Fortescue River on the Pilbara Coast and krill at Town Beach in Exmouth in the state's north-west in recent weeks.

Wild oysters were found dead on November 15 after a fish kill near Karratha. ( Supplied: WA Department of Primary Industries )

The South West was not immune either, with about 800 dead abalone and other shellfish species including crabs and various molluscs found along a beach stretching more than a kilometre north of Yallingup.

The Department of Primary Industries and Regional Development said it had carried out a range of tests to rule out other potential causes.

Hundreds of abalone were found dead in the South West beach of Yallingup on December 2. ( Supplied: WA Department of Primary Industries )

"According to our standard fish kill response protocol, samples have been taken and analysed and we have been able to exclude infectious diseases and found no evidence of significant algal blooms associated with these events," said the department's fish kill coordinator, Marion Massam.

"Therefore, the recent fish kills may well be a consequence of changes in environmental conditions."

Heatwave of unprecedented size

The scale of the marine heatwave gripping the WA coastline was unprecedented, according to Professor Pattiaratchi.

"We haven't actually had one before that extends all the way from the Kimberley to South Australia," he said.

But it was far too early to compare its intensity to the extreme marine heatwave of 2011, which had devastating ecological consequences.

Sea surface temperature percentiles for WA on December 14, 2018. They are much cooler than those recorded one year later (below). ( Supplied: Integrated Marine Observing System )

Sea surface temperature percentiles on December 14, 2019. Much of the WA coastline was warmer than the 90th percentile for this time of year. ( Supplied: Integrated Marine Observing System )

That event caused widespread coral bleaching and fish kills at two UNESCO World Heritage Sites — Shark Bay and the Ningaloo Coast.

"We are not 100 per cent sure whether this is only a short-term thing — for heatwaves to become extreme they have to last longer," Professor Pattiaratchi said.

The temperatures the ocean off the west coast is currently experiencing are not as high as the 2011 heatwave, which occurred in February when temperatures are historically much warmer.

"It's warm compared to a normal December but it is not as warm as what we've had previously in other months," Professor Pattiaratchi said.

What causes a marine heatwave?

The warming trend in the oceans could be attributed to high air temperatures, a lack of cloud cover and weaker winds.

It will take strong southerly breezes to abate the warming pattern but they are not forecast any time soon.

"We need two or three days of really strong southerly winds … more than 25 knots," Professor Pattiaratchi said.

"Between now and Christmas we will have sea breezes but they're less than 20 knots."

2019 is tracking to be up there to be up there with 2013, Australia's hottest year on record. ( Supplied: Bureau of Meteorology )

He said it was important to note the ocean surrounding WA had been generally cool over the past couple of years, while the Tasman Sea has had back-to-back heatwaves.

But now the tables had turned.

"We are in the top 10 per cent of warm years, Tasmania is in the bottom 10 per cent … they have the exact opposite of what we're experiencing," Professor Pattiaratchi said.

"That has usually been the case in the last few years, that when south-east Australia has warm conditions we've had cold conditions and vice versa."