THE resumption of grain harvest in northern Victoria and parts of NSW has confirmed growers' worst fears about the impact of last month's torrential rain.

Grain growers in northern Victoria and parts of NSW are facing devastating yield losses in a season that has "thrown everything" at them.

Rain yesterday was expected to further delay the harvest in Victoria.

The storms followed dismal winter rainfall, frosts in some areas and a heatwave in early November that has cut yields by up to 100 per cent.

NSW Farmers Association grains committee chair Mark Hoskinson said farmers in the areas around Young and Greenthorpe were facing 100 per cent loss in some paddocks due to last season frosts.

Mr Hoskinson said yields were half what they were last year due to a lack of rain during winter and scorching temperatures early November on his central NSW property.

"We started harvesting a fortnight earlier than usual, but that wasn't because the crops were ripe, it was because they were cooked," he said.

Mr Hoskinson said he expected 75-80 per cent of grain would be warehoused, with a small amount being sold immediately for cashflow.

Further south at Lockhart Andrew Bouffler said the season was "pretty ordinary."

"There was massive moisture stress, then frost and a hot spell," Mr Bouffler said.

"The season has thrown everything at us, so to get one tonne to the hectare is good."

He said while his crops were not badly affected by 37mm of rain in late November, he'd heard farmers at Rennie, further south, were facing shot grain.

In Victoria, St Arnaud farmer Colin Coates said his wheat had suffered from the 85mm that fell in November.

However, he said he was pleased to be busy.

"In the drought years, you're not busy and you wish there was something to harvest," he said. "We're busy now and we'll think about marketing it later."

He said the yields were good considering the poor season.

Rupanyup grain grower Andrew Weidemann estimated he had lost 20-25 per cent of his yield due to the heat and the rain.

"It's certainly not the year we thought it would be five or six weeks ago," he said.

Mr Weidemann said growers were racing to finish as much harvesting as possible before forecast rain this week.

Meanwhile, ABARE has cut its estimate of national grain production from 36 million tonnes to 35.7 million tonnes.

Total wheat production was revised down from 22.7 million tonnes to 22 million tonnes.