"The Electra Trust operates under a Trust Deed created within parameters set by the government in 1993," said Mr Turver.
"The Deed requires that, for a consumer-owned lines company, all surpluses be returned to their consumer owners.
"The law has not been changed in that time, so why has the IRD decided to take this approach now? There was no forewarning."
Mr Turver said the discount was not a commercial dividend and consumer-owned trusts are using exactly the same principle as large commercial firms which discount products using 'cash backs' for purchases and don't get taxed on the cash back value.
"The fact [IRD] have put something in writing is as good as saying something is going to happen," he said.
"I hope power consumers will voice their concerns, and I urge them to write to the chief executive of Inland Revenue."
IRD have so far refused to comment to media, citing the secrecy provisions of the Tax Administration Act.