A record-breaking cold spell in Western Australia's agricultural heartland has shattered farmers' hopes of enjoying a bumper crop harvest.

Temperatures in the southern Wheatbelt dipped between 0 and minus 5 degrees Celsius on both Saturday and Sunday morning, while the Great Southern town of Newdegate recorded its lowest-ever temperature of minus 4.3 degrees.

Farmers woke to discover heavy frost gripping their wheat and barley crops, which were expected to deliver a combined 14 million tonne harvest this year.

Kojonup farmer Dan Ladyman, whose property is about 260 kilometres south-east of Perth, said the frost was fierce and widespread.

"First impressions was that it was obviously pretty severe," he said.

"It wasn't like the shadows were just frosted, it was a complete white out."

Farming properties in Kojonup were hit hard by frost. ( Supplied: Dan Ladyman )

He said any significant damage to his crops would become apparent in the coming days.

"We probably expect to see a bit of damage in the canola and lupins that have been flowering, and hopefully nothing with the cereals because they aren't flowering yet for us," Mr Ladyman said.

"For us, in another two of three weeks when we're flowering is when we definitely don't want to be having (frost).

"But that's the game that we're in — we've just got the learn to be able to ride it out if it does happen."

Nyabing farmer Scott Crosby reported major damage on his cereal crops.

"It's not going to be a fun harvest," he said.

Windscreens in Narrogin were covered in frost. ( Supplied: Miranda Lee )

"There was some comment that it wouldn't be too bad because the crop is late, but not much likes minus 5 degrees regardless of the crop stage.

"What we need is rain ASAP to help reduce the damage. If it stays dry, the damage will be great."

Bureau of Meteorology duty forecaster Darryl Vink said a less severe frost event was expected in the coming week.

"We've got a cold front approaching the South West this evening — so a bit more wind and bit more cloud preventing the risk of anything extreme in terms of frost," he said.

"But then there is another high ridge of high pressure developing with cold wind, light winds and clear skies on Tuesday morning.

"So there's a moderate frost potential through the central Wheatbelt and Great Southern on Tuesday morning."

In Perth, temperatures fell to 2.8 degrees on Sunday morning, the coldest September low since 2015.

Jandakot temperatures slumped to 1.9 degrees, while the mercury reached just 1.6 degrees in the Swan Valley.

Perth's coldest ever September day was in 2008, when the temperature plunged to 1 degree.