The decision by the federal Department of Defence to veto a joint venture deal between miner Western Plains and Chinese steel mill Wuhan Iron and Steel sent Western Plains falling to its biggest one-day slump in almost six months. A 15.6 per cent fall left the shares of the iron ore company trading at a five-month low of 27¢ on Wednesday. The proposed $45 million deal involved selling the Chinese company a 50 per cent stake in the Hawks Nest magnetite project located in the Woomera Protection Area in South Australia, but was quashed on "national security grounds". Some fear the decision could risk damaging Sino-Australian relations further given it follows the government's thumbs down to OZ Minerals to sell its Prominent Hill copper mine to Chinese interests. Western Plains has fallen by 42 per cent since mid-May and is a far cry from its record high of $1.84 from December 2007. Small cap manager Contango is the second largest shareholder in the stock with a 7.45 per cent stake.