National faces a backlash from heartland supporters over the sale of the Crafar dairy farms to a Chinese buyer, with many vowing they will never vote for the party again.
Government ministers approved the sale of the 16 North Island farms to Milk New Zealand Holdings, a subsidiary of Shanghai Pengxin, in January.
Many National backers have written to Prime Minister John Key to express their distaste for the deal, with more than 100 emails or letters opposed to the sale, the Sunday Star Times reports.
"I personally feel that such sales are an act of treason and gross betrayal of trust and will not be voting National next election," one letter says.
Another reads: "As far as I am concerned the National Government is selling us down the river. I have been a National Party supporter for 45 years and my family before me. Never again.
"The agreement to sell the Crafar Farms to Asians when New Zealanders wanted to buy them is the final nail in the coffin for me."
Meanwhile, it has also been revealed that the farms were offered for sale individually in Asia, despite potential New Zealand buyers being told the farms could only be sold profitably in a bulk deal.
The advertisements, in the South China Morning Post and Singapore's Straits Times, were released to the Sunday Star Times by the Overseas Investment Office (OIO) under the Official Information Act.
The ads described the farms as "one of the largest and most highly anticipated rural portfolios to ever come to the market in New Zealand".
"A significant opportunity to invest in New Zealand's rural and dairy sector. Farms are for sale individually or as a portfolio."
The Shanghai Pengxin deal - reportedly worth $210 million - remains on hold after a rival consortium, led by businessman Sir Michael Fay and offering $171.5m for the farms, went to the High Court to overturn the decision.
Justice Forrest Miller ruled that the ministers "materially overstated" the economic benefit of the transaction to the New Zealand economy and they must reconsider Milk New Zealand's application.