Why the humidity on Australia's east coast is worse than EVER: Expert explains reason parts of the country are sweating

Millions of Australians are currently sweltering through high levels of humidity with a meteorologist describing the phenomenon as 'unusual'.

Large swathes of the country have been battling through muggy weather with humidity levels regularly pushing past 90 per cent overnight in cities such as Sydney, Melbourne and Brisbane.

Coastal NSW and Victoria, inland and northern Queensland and large swathes of Western Australia have been hit the hardest.

Weatherzone meteorologist Michael Reeder said months of easterly and northeasterly winds coming from warm waters has led to higher humidity.

'It's been moist for quite a while now, for most of the last couple of months hasn't it, there's been it's been a lot of a lot of rain,' Professor Reeder told Daily Mail Australia.

'Now, one of the things that's been noticeable over the last couple of months is there's been a pretty constant easterly trough.

'This is important, it's important because it's bringing moist air onshore from over the ocean.'

Professor Reeder said the trough had been produced by a historically warm spring, the fifth-hottest since records began in 1910. 

While southerly winds usually sweep through and replace the trough, they are instead merging and causing moisture to be 'dragged towards the southern part of the continent'.

Professor Reeder said Cyclone Jasper also had a hand in humidity levels due to the direction of its spiral.

'The airflow around the tropical cyclone is clockwise in the southern hemisphere, so that means on the southern side of the cyclone, you're gonna get stronger easterlies,' he said.

The stronger easterlies and north-easterlies produced by Cyclone Jasper have further exacerbated the moist conditions.

'It can produce moisture conditions inland, north and south of it, it can and it has,' Professor Reeder said. 

He said the spike in humidity was unusual because of how early summer-like conditions hit Australia.

'Because the conditions have been building, it's hot and the trough formation, it's  become wetter,' professor Reeder said.

'This is early in the season, later in the season when you normally get these troughs forming you get wet season in northern Australia and in southern Australia, it's dried out a lot.

'Now, of course, all this has come after it being hotter earlier on than usual. 

'We've got the trough, and it's sort of wet in the north, but also it's quite moist in the south as well.

'It's not common. It's not the way it usually plays out, but I can think of plenty of occasions when it's played out like this.'

Muggy conditions are expected to remain on the east-coast during Christmas as the Bureau of Meteorology predicts rain and storms in the lead-up to the holiday period.

Showers and cool temperatures are forecast for Sydney, Brisbane, and Canberra, while Melbourne, Adelaide, Darwin, and Perth will remain mostly dry and warm.

Heatwave warnings lasting until Tuesday remain in place for large areas of WA, southeastern Queensland, coastal and northern NSW, and parts of the NT.