The unexpected deluge made it difficult to plan ahead, he said.
"The forecast wasn't for this; it was forecast for about 25 mm of rain overnight, so to have ten times that amount is a little bit hard to manage for."
He had already lost "a fair few ewes" out of a river paddock further towards town.
"I obviously didn't know the forecast was for this amount of rain and they were sitting in there waiting for a truck this week ... the person living down there sent me a few videos of them floating a little bit in the river.
"So I don't know my losses yet but I'll find out in another couple of days when I can get there."
Slips were also an issue, due to a "pretty typical old-fashioned winter," Firth said.
"It's wet, cold and that soil moisture level's so high it doesn't take much rain to fill 'er up and get it all running and flowing off and obviously the ground can only handle so much before it starts moving."
He said the ground began to shift about three weeks ago, causing slips, wiping out fences and even damaging the road.
"[There's] slips along the road, a few washouts … and further inland it's ripped up the tarseal as well."
At the time of the interview, the river still hadn't receded, he said.