Perth snowman
© Mark Fowler/TwitterA farmer made this snowman from frost at his Williams property yesterday.
If you thought it was unseasonably cold in Perth this month, you weren't mistaken.

The Bureau of Meteorology has confirmed what everyone in the city was thinking this month - that it was much colder than usual for this time of year.

It was the coldest September since records started in Mount Lawley - where the city's weather station is based - with temperatures reaching an average of just 18.5 degrees Celsius.

Records have been held at the station since 1994.

The temperature is almost three degrees colder than the usual average temperature of 20.4C.

Overnight, an extra blanket was a must with the average temperature just 7.6C - two degrees colder than usual.

At Perth Airport, where records stretch back decades, overnight temperatures haven't been this cold in September since 1944.

And there was very little relief.

The mercury rose above 20C just four times this month, reaching a maximum of 23.1C on only one day.

Snow has been forecast for the Stirling Range National Park this weekend and frosts has devastated crops in parts of the Wheatbelt.

Farms around Bruce Rock and Hyden are among the hardest hit.

The cumulative effect of multiple severe frosts from mid-August to the end of September wiped out whole paddocks.

Grain grower-controlled storage and handling business CBH had revised the upper end of its harvest estimate down from 17.6 million tonnes to 17mt as the frost damage becomes clearer.

The cold, wet September will delay the start of harvest in much of the Wheatbelt as farmers wait for crops to dry out and mature.

The Bureau's media and communications manager Neil Bennett said cold fronts were to blame for preventing warm winds coming in from the north.

"I think it's a continuation of the trend of cold fronts coming through," he said.

Since June, there have been only 20 days where the temperature in Perth has been 20C or higher, compared to an average of 45 days.