West Australian grain growers are counting the cost of a massive thunderstorm carrying rain and hail to the state's South West.

Lake Grace farmer Mae Connolly told 720 ABC Perth her barley crops would suffer a drop in quality after being flattened by heavy rain.

"That's going to have a big impact on our crop quality," she said.

"They were about to be harvested, so it's too late for the rain to do any good whatsoever."

Ms Connolly said moving floodwaters had also caused damage to canola crops.

"With canola, we cut it off at the stalk and lay it on the ground for a couple of weeks, so flood water moving through is not great for that," she said.

"We've gone from really good looking crops, we pride ourselves on quality in our crops, and that has been lost, there's a real threat that they will be downgraded to feed quality."

Ms Connolly said what was expected to be a bumper crop would now be unprofitable.

"Prices have fallen over the years, probably most people were in the position that prices were low, but yields were looking good enough that this year was going to be ok, it was going to be profitable," she said.

"But suddenly, in a few hours yesterday, we've gone to potentially a lot of feed grain, which is just not going to be profitable, those crops will now run at a loss."

Ms Connolly said no-one could see the heavy rain coming.

"With a drought you see it coming, it builds up over months, but a flood coming right on the doorstop of harvest, I just absolutely feel for everyone that's been affected by this," she said.

Yorkrakine farmer Quentin Davies said the weather events had caused "massive damage" so close to harvest.

He said rain and hail had cut a swathe through crops across the region, about 10 kilometres wide and 150 kilometres long.

"It's probably cut a path from, what I can gather Goomalling, south Dowerin, south Wyalkatchem, out through Yorkrakine and headed off east," he said.

"There was some pretty significant rainfall events, in some places in excess of 50 millimetres, and I think it was the time in which it fell- 20 or 30 mils in less than half an hour is a lot of rain."

Mr Davies said while insurance could cover some losses, the situation was disappointing.

"We can insure against hail, which I'm hoping everyone has, so we'll get out of that OK," he said.

"It's a bit disappointing that you've put the work in and you've had a good crop sitting out there and you don't get the pleasure of going to harvest it.

"But there's guys further east here that really would have loved to have a crop to insure, and they haven't even had one, so I guess mother nature, we can't ring anyone up and I think that's what people want to do, they want to blame someone, the bloke we've got to ring up and blame for this, I don't think he's got a phone number."

"We've all got choices and we've made a choice to be farming, we certainly can't control the weather, that's just part and parcel out here."

Source - http://www.abc.net.au/