Northern NSW macadamia farmer Warren Elvery said the damage was devastating.
"There's one dairy farmer that lost 180 head, there's beef producers out getting the cows in on horseback," Mr Elvery told AAP.
He said huge dumps of rain at his property at Dalwood, between Lismore and Ballina, caused instant flooding.
Mr Elvery, who had planned to harvest next week, said up to 15 per cent of his macadamia crop was wiped out.
Towns across northern NSW, including Lismore and Ballina, are underwater and rivers are peaking. Further north, many Queensland communities have also been devastated by the inundation.
Jo Sheppard, chief executive of the Queensland Farmers' Federation, said the impact had been "major to catastrophic" around parts of the Lockyer Valley, Sunshine Coast, Brisbane and the Gold Coast.
"It's looking grim," Ms Sheppard said.
She told AAP some dairy farmers were unable to get feed to their properties or get their milk out to producers because roads had been cut.
"Farmers are still trying to get back to their farm because of the road closures, so it's really hard to get a handle on it."
The deluge hit parts of Queensland just as the state's summer crops were being harvested.
AgForce grains president Brendan Taylor told AAP that grain farmers are "gutted" because the headers had just started, with sorghum expected to be the worst-hit crop.
"It will attract a significant discount on the market," said Mr Taylor, who is in Queensland.
He said some of the crops that get planted later in the season, like mung beans and cotton, would benefit from the moisture.
Cattle farmer Sid Plant said the rain had washed away fences and caused thousands of dollars in damage at his 3000-acre farm near Toowoomba in Queensland.
It's the fifth time his property has flooded in as many months.
Mr Plant, who is part of the Farmers for Climate Action group, said the floods feel like the "last warning of the freight-train coming that is climate change".
