Union leader John Setka's assistant secretary, Shaun Reardon, has quit over "irreconcilable differences" with his boss relating to "personal values I will live and die by".

Mr Reardon emailed CFMMEU staff, officials and members on Thursday about his decision, which comes in response to Mr Setka admitting he would plead guilty to criminal offences of harassing a woman and repeatedly breaching court orders.

CFMMEU leaders John Setka and Shaun Reardon appear at the Magistrates Court on blackmail charges. Credit:Justin McManus

Mr Reardon, until today the assistant secretary of the CFMMEU’s Victorian construction division, has been a long-term campaigner against violence against women and domestic violence.

In the email to staff, obtained by The Age and Sydney Morning Herald, he said the decision to leave was his own.