Mrs Hosking said proposed new nutrient discharge rules had been a huge factor behind the drop in value.
Part of the Rotorua Te Arawa Lakes Programme, the rules aim to ensure the long-term water quality of Lake Rotorua by limiting land users' nitrogen discharges. "The impact of the rules mean the land is no longer economic and value has been halved.
"Everybody is very upset about the new rules and now these new valuations have turned everything upside down for me," she said.
Proposed nitrogen discharge allowances for the farm - between 10-20kg/ha - would restrict stock numbers.
Central Rd farmer Janet Arnet said she was "astounded" how much the rateable valuations had dropped - in her case 30 per cent.
Rotorua rural sales agent Chris Meban said vendors were concerned about the new rating values.
"I would like to know how the figures can be substantiated," he said.
"My biggest concern is the inconsistencies from property to property, and the amount of equity that land owners are losing with a stroke of a pen."
Mr Meban said sales prices had been close to 2011 rateable values - "some above and some below".
"Some of my clients have had drops of more than 20 per cent."
He advised owners to make strong objections to the council.
The triennial rateable valuations were assessed by an independent valuer, based on recent sales data and property inspections.
Rotorua District Council chief financial officer David Foster said pastoral farms, residential, commercial, industrial and lifestyle blocks had all gone down in value, while dairy farms had seen a slight increase.
"Rural values have not dropped overall," Mr Foster said. "On average, there has been an 0.1 per cent increase, with dairying up 20 per cent, forestry down 4 per cent and pastoral down 8 per cent."
Latest valuations had reflected prevailing recent market conditions, while rural sector valuations were affected by fluctuating dairy milk solids payout, and the impact of restrictions on nutrient discharge, he said.
The new figures showed a decrease in residential values but there had been a 0.1 per cent rise - to $12.88 billion - in the total capital value of all 29,873 properties in the district since 2011, he said.
Owners have until November 17 to object to new valuations.
The new Rotorua District Valuation Roll is open for public inspection at the council's offices in Haupapa St and online through the Rating Information Database at rdc.govt.nz.