AGL boss Brett Redman is leaving the energy provider after only last month starting efforts to create separate coal-fired power and clean energy businesses.
The company on Thursday said Mr Redman had resigned after two and a half years as CEO, effective immediately, as he could not commit beyond the proposed demerger.
AGL plans to have one business offering low carbon-emitting technologies, providing consumers electricity, gas, internet and mobile services. The other will continue operating coal-fired power stations to generate electricity.
The AGL board wanted alternative leadership to guide the demerger once it became aware of Mr Redman's situation.
Mr Redman had made the initial case for the structural separation of the business.
Chairman Graeme Hunt will stand down to become interim managing director and chief executive.
Non-executive director and former Oil Search CEO Peter Botten will take the chairman role.
However, Mr Redman will remain "available" to AGL until the expiry of his notice period on October 21.
AGL shares were down 2.32 per cent to $8.85 at 1108 AEST.
