A weak frontal system delivered patchy rain to many areas, resulting in winners and losers.

Despite good rain in some areas increasing yields there are new fears that southern Wheatbelt frosts will take away the increased production.

CBH operations manger Michael Musgrave says however, WA's forecast grain production this season has not changed from the 9-11 million tonnes estimated last week.

"Our position hasn't changed despite the fact that there has been good rain," Mr Musgrave said.

"We are keeping the forecast at 9mt to 11mt because we have had reports that there has been frost in the southern half of the Wheatbelt.

"Until we confirm the reports, which will take a while, the forecast will stay at this level.

"Even though the rain will increase yields we aren't increasing the range because of frost damage."

Mr Musgrave said it has been a difficult year for harvest planning due to the intermittent rain.

Agriculture Systems research manager David Bowran said the rain has been extremely patchy and variable.

While some areas close to the coast, such as Badgingarra received 27mm last week, further inland, falls have been inconsistent.

"Three Springs received 5mm while Coorow had 6mm," Dr Bowran said.

"There has been very low rainfall in the central Wheatbelt and it has been variable.

"For example, Merredin had 16mm while Dalwallinu received 5mm.

"In the Great Southern, some areas received almost no rain, with high variations like those at Darkan which recorded 19mm."

While some people were thinking and hoping this rain would be their saviour, they were already seeing crops that were drying off, Dr Bowran said.