The party said Westland's processing plants were on land that was arguably used for an agricultural purpose and was therefore farmland.
The OIO may have failed to apply a much stricter set of criteria under the Overseas Investment Act 2005 which would have seen ministers, not delegated officials, make the final decision, Social Credit said.
But Justice Robert Dobson ruled that the interpretation of farmland did not extend to sites used for processing raw milk and further manufacturing and processing activities.
Therefore, the Ministers of Finance, Fisheries, and Land Information NZ (Linz) were correct to delegate the responsibility of making a decision on Jingang's application to the chief executive of Linz.
Leitch said his party was considering appealing the High Court ruling but no decision had been made as yet.
Social Credit said it wanted to see New Zealanders reaping the benefits of the nation's world leadership in the primary sector, not overseas shareholders.
The party said massive chunks of the overseas exchange the country earned from its world leadership were lost when overseas owners claimed their profits.
There was a tidal wave of applications from overseas entities that wanted to snap up the best agricultural land and agricultural and horticultural businesses that New Zealand has, so the party was taking a stand on the issue, Social Credit said.