In parts of New South Wales some crops suffered such "severe moisture stress" that they have already been grazed by livestock, or "sprayed out", according to a new federal government report that predicts the winter crop production will be limited to 36.3 million tonnes.

Winter crops were in a "very poor condition" at the start of spring in parts of New South Wales, Queensland and Western Australia, according to the report from the Australian Bureau of Agricultural and Resource Economics and Sciences, that "it is likely many will not be harvested".

And after many growing regions of New South Wales had a very dry winter, repeated frosts in August caused more damage to stressed crops.

In late July the president of NSW Farmers Association, Derek Schoen, summed up the impact of the dry conditions when he told Fairfax Media that on some NSW farms "you've got some crops that are actually turning blue, which is an indication of severe moisture deficiency".

On Monday he told Fairfax Media that things had got "considerably worse" since then.

"It's a very disappointing season. We had a patchy start and then it got very dry in some areas," he said.

According to ABARES, winter crop production in NSW is tipped to fall 43 per cent to about 9.5 million tonnes.

"The next two weeks are going to be absolutely pivotal in finding out exactly what the damage has been from the frosts, and what the final yield prospects are going to look like. But we're still reliant on the spring rains coming then to finish any crops that haven't been frosted," he said.

The ABARES report, released on Tuesday, says that the fall in nationwide harvest is largely due to a fall in average yields this season, after the exceptional yields during the record harvest of 2016-17.

Total production is actually expected to be up about two per cent on the 10 year average to 2015-16.

This season's forecast crop is down substantially, nine per cent, from the previous ABARES report produced only about three months ago.

Total wheat tonnage is expected to fall 38 per cent to 21.6 million tonnes, barley production is tipped to fall 40 per cent to 8 million tonnes and canola production is forecast to fall 33 per cent to 2.8 million tonnes.

"These forecasts will only be achieved if spring rainfall is sufficient and timely, especially in central west New South Wales and the Eyre and Yorke peninsulas in South Australia," said ABARES' chief commodity analyst, Peter Gooday.

While farmers in parts of New South Wales have already written off some of their grain crops, things are not as bad in Victoria, although ABARES tips Victorian winter crop production to fall 34 per cent to 6.8 million tonnes.

"In the traditional cropping areas we've seen, even with the dry weather, we've seen good rainfall, both opening rains and now follow up rains in late winter. That has enabled most of the crops to set some really good potential," said the president of the Victorian Farmers Federation, David Jochinke.

But in the western districts, near Hamilton, conditions were "a touch too wet, which is having an effect on some of their yields," he said.