The letter was in response to the current trustees wanting to phase out TECT cheques, starting with a one-off $2500 cheque later this year plus five more annual cheques of $360 each.
Current trustees have proposed putting all the annual returns from its Trustpower shares and other investments into the TECT Charitable Trust that was set up in 2002 to ensure the share of the money paid to community organisations was not taxed.
The letter from the former trustees said: "Nowhere have you indicated the grounds upon which you consider this proposal to be in the best interests of consumers.''
Their letter also said current trustees had provided no valid reason for forming the view that doing nothing was no longer an option for TECT.
''All this, however, is not relevant to your proposal, which is that consumers give up their entitlement to future distributions and agreed to the transferring or distributing the bulk of TECT's assets to TECT Charitable Trust.
"Indeed, you say that TECT has no plans to sell its Trustpower holdings. We regret we are left with the impression that the waters have been deliberately muddied by your statements,'' the letter said.
TECT's current chairman, Bill Holland, said that under the proposed changes TECT had no short-term intention to sell-down its Trustpower shares. But it could diversify its investment portfolio in the medium to long term - over five to 10 years.
He said the views expressed in the letter were in complete contrast to the views of two other previous TECT chairpersons, Noel Pope and Jan Beange, who supported the new direction.
Holland disagreed trustees had given no valid reasons for the change, saying changes were already underway such as solar energy and battery technologies and people buying their electricity with a card; industry structural changes such as takeovers; and regulatory changes such as Governments intervening in electricity pricing.
He questioned whether reducing the amount of TECT cheques would be the best response to these pressures. ''Or should we be proactive and make changes on our own terms?''
Holland said a lot was being made of 55 years left to go for the trust and the size of the offer to consumers.
He said being a TECT consumer was not transferable and it was only while they were Trustpower customers who lived in the Western Bay. Anything could happen over 55 years - people could die, move away or change electricity retailers.
Credentials of former TECT trustees
Michael Cooney: Former TECT chairman who also served on the board of Trustpower. Retired as trustee after 16 years in 2014.
Bruce Cronin: Former deputy chairman who served as a trustee for 20 years. Did not seek re-election in 2016.
Ken Collings: Served at least eight years as a trustee.