One of the nation's peak mining unions has urged the Coalition to pull into line its backbenchers who are mounting "undiplomatic, reckless and sometimes bizarre'' attacks on China, saying their "pointless machismo" posed a grave threat to coal exports.

Tony Maher, the general president of the CFMEU's Mining and Energy Division, has written to Resources Minister Keith Pitt warning that thousands of coal miners could lose their jobs before Christmas.