Frost damage is continuing to wipe millions of tonnes off Western Australia's expected grain harvest.

The state's main grain handler and marketer the CBH Group has revised its harvest estimate from 15–17 million tonnes down to 13–14 million tonnes.

CBH Group general manager of operations David Capper said feedback from grower meetings and agronomists had pushed the expected tonnes down.

"But in reality, even the sources of information that we're getting won't really know what their estimates are going to be, or what their crops are going to be before the header goes in," he said.

"Growers can only give us the best information that they've got based on what they're seeing in their paddocks. The header will be the deciding factor."

A harvest of 13–14 million tonnes is still well above the average harvest of 10.2 million tonnes.

"It's on par with the past couple of years, but unfortunately not the larger crop that we were anticipating earlier in the year," Mr Capper said.

He said the frost damage was widespread across the state and as harvest began, frost damage was appearing in areas where it was not anticipated.

He said CBH was introducing new segregations for barley, which was badly affected, and was looking at introducing lower weight segregations for wheat.

"Prices are depressed as it is, and lower quality grain is going to be more so," Mr Capper said.

"It just makes that challenge of ensuring that we don't force growers to downgrade grain that doesn't need to be downgraded by not having the right services available.

"But at the same time giving growers the opportunity to deliver their grain that has been badly affected by the seasonal conditions."

Mr Capper said frost was one of the hardest seasonal conditions to deal with.

"You've gone through the season, you've spent the money, you've fostered and grown and developed what looked to be a fantastic crop and spent the money to do that, and then you lose it at the end," he said.

"That's a really difficult thing to deal with, but it's great to see the communities coming together and supporting each other."