Australian farmers will harvest their smallest national grain crop in 12 years, according to the official forecaster.

Key points: Below average rainfall and daytime temperatures above average in most cropping regions have reduced crop prospects

Below average rainfall and daytime temperatures above average in most cropping regions have reduced crop prospects In Western Australia and southern New South Wales the reductions were the most significant

In Western Australia and southern New South Wales the reductions were the most significant Yields in most southern cropping regions in South Australia and Victoria are expected to be average to above average

Dry and hot weather in the major grain-growing states of New South Wales and Western Australia has slashed the national winter crop by 13 per cent from the spring forecast.

The Australian Bureau of Agriculture and Resources Economics and Sciences today released its latest crop report and is predicting a 29.4 million tonne national winter crop.

This is about 27 per cent less than the 10-year crop average and is the third year in a row of declining crop production.

The Australian wheat crop has slumped 8 per cent from last year's drought-affected production year to 15.9 million tonnes, the smallest crop since 2007-08.

Frost, heat and dry kills crops

Harvest in Western Australia has been disappointing after low rainfall in most regions, and heat stress in spring. ( ABC Esperance: Emma Field )

ABARES acting executive director Peter Gooday said the downgrade was a result of poor spring conditions.

"Below-average rainfall and above-average temperatures during spring reduced winter crop prospects in most cropping regions," he said.

"But the changes in Western Australia and southern New South Wales had the biggest impact on national production prospects."

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The situation is dire in New South Wales. Its wheat crop is tipped to be 2.2 million tonnes. ABARES wiped almost a million tonnes from its September report and this figure is 67 per cent lower than the state's 10-year average.

"The central and northern parts of the cropping zone has been pretty bad right from the start of the season, they have had drier than average rain right the way through," ABARES senior economist Peter Colins said.

Many farmers decided to abandon crops and cut them for hay in spring ABARES reported, further downsizing production.

Western Australia takes a hit

The nation's largest export state Western Australia has seen the most dramatic change.

Last year it produced a bumper 10 million tonne wheat crop, that has been cut to a 6 million tonne crop this season.

Extreme weather in WA, including several unprecedented spring frost events, and poor seasonal rainfall meant ABARES slashed 2 million tonnes off the state's estimated wheat crop from its spring report.

"Heat stress and lower moisture adversely affected most winter crops in the state, especially in northern cropping regions such as Geraldton," the report said.

Better news in Victoria and South Australia

The bright spot in the new report was Victoria, which will almost double its winter crop production from last year's drought-affected year.

ABARES is forecasting the state will produce a 7.2 million tonne winter crop, which includes a 3.6 million tonne wheat crop.

Meanwhile crop production in South Australia is tipped to be up by about 19 per cent on last year, but it will still be below the long term average.

Southern Cross farmers Ron and Ryan Burro cut back their cropping program after a poor start to the season ( ABC Esperance: Emma Field )

WA farmer scale back crops

Ron Burro and his son Ryan farm at Southern Cross in the far eastern part of the WA wheatbelt, about 400 kilometres from Perth.

Their crops were below average this year, his wheat yielded between 0.9-1 tonne a hectare, but his barley was worse.

And it could have been worse if they had not scaled back when autumn rains did not arrive.

"Nearly half of our program got cut because of the way the season was looking, the rain was far too late," Mr Burro said.

"(We) farm for the environment and our own survival."