"We believe the system [which was linked to customers' electricity usage] is fair, and for many years the tax man agreed," Mr Shaw said.
"Now they seem to be viewing it as a dividend rather than a discount."
Last week's Sunday Star Times, which revealed the memo, quoted Wellington-based PriceWaterhouseCoopers partner Todd Stevens as labelling the proposal as outrageous.
Hundreds of thousands of families would lose tens of millions of dollars, he said, asking if the IRD's next move would be to tax Briscoes' weekend discounts.
He estimated taxing $87 million in rebates would deliver $24.4 million to the IRD.
For more than 15 years, he added, the rebates had been an important mechanism for lowering household power bills.
Inland Revenue declined to discuss the affairs of a "small group of taxpayers," citing secrecy terms of the Tax Administration Act.