AGL Energy has walked away from a $3 billion takeover of Vocus Group, leaving investors relieved it has dropped a perceived high-risk deal but questioning how else it can arrest an expected multi-year decline in earnings.

Just five days into a four-week exclusive due-diligence period on the fibre-optic cable owner, AGL's relatively new chief executive, Brett Redman, said the company was "no longer confident that an acquisition of Vocus at the proposed terms would represent sufficient certainty of creating value for AGL shareholders".