Leitch said his proposal wouldn't cost taxpayers a cent and is not a new concept.
Governments had enabled overdraft facilities through the central bank for New Zealand producer boards from 1936 up to the mid-80s, he said.
"We're saying why are you (Westland) going to an overseas entity to obtain (needed) capital when you could do it through our own central bank, keeping ownership with Westland farmers and ensuring they continue to have a New Zealand company to supply milk to but also the dividend flows from it.
"Whereas the sale to Yili will see all that going overseas - and adding to our balance of payments problem."
In his letter to the ministers, Leitch said the potential for significant job losses at the West Coast's biggest employer after the sale to Yili should be a cause for concern.
"What should be ringing alarm bells louder however, from New Zealand's economic and strategic perspective, is the potential for this to be a stepping stone to the control by the government of another country of our entire milk processing capability, giving them the power to dictate the price that New Zealand's milk producers will receive for their product.
"This would ultimately lead to many New Zealand farmers selling their land to the overseas buyers, likely hailing substantially from mainland China, who are keen to acquire New Zealand farmland. That would prove disastrous for New Zealand's economic future.
Leitch wrote that the economic benefits of trade with China would "disappear when they have vertical control of our biggest industry".
The concept of using the central bank to fund some government expenditure at no cost to taxpayers was used by Japan and China and was supported by international economic experts including Lord Adair Turner, former chairman of the UK's financial services, wrote Leitch. A growing number of international and New Zealand economists and commentators also supported it.
Leitch said he'd had no response from the Beehive.
Robertson's office has been approached for comment.
Social Credit has been a registered political party since 1953. It linked with Jim Anderton's Alliance party for a while and then rebranded under Democrats for Social Credit, before last year deciding to return to its original name. Leitch was elected leader last year.
He said the party was now selecting candidates to contest next year's election. He hoped it would field 15 to 20 candidates.