The Abbott-Turnbull government was warned about the potential of consumers being gouged by electricity networks for more than three years but failed to act until now, letters obtained from two state governments show.

Federal Environment and Energy Minister Josh Frydenberg announced on Monday that networks and gas pipeline operators would be investigated by the energy watchdog - the Australian Energy Regulator (AER) - for potentially overcharging consumers by $400 million a year to cover their corporate tax bills.

Federal government stands accused of knowing about the overcharging of consumers for years before acting. Credit:Michael Howard

Letters obtained by Fairfax Media, however, show the federal government was warned as far back as February 2015 by South Australia's energy minister Tom Koutsantonis about the networks' income tax allowance that permitted the overcharging.

Lily D'Ambrosio, Victoria's energy, environment and climate change minister, wrote in a separate letter to Mr Frydenberg last month about the same issue.