By Renju Jose and Christine Chen

SYDNEY (Reuters) -Australia on Friday blocked a A$672 million ($435 million) buyout offer for Mayne Pharma from Cosette Pharmaceuticals after the U.S. drugmaker became a reluctant suitor and threatened to close a local plant.

The decision sent shares in Mayne tumbling 23% before they were put on a temporary trading halt.

Treasurer Jim Chalmers said his decision was entirely consistent with advice from the Foreign Investment Review Board that the proposal would be contrary to Australia’s national interest.

“This is about doing what is necessary to protect Australia’s national interest, the security of our critical medical supply chains, local jobs and the local community,” he said in a statement.

Mayne makes both branded and generic drugs, focusing on w

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