Napier mayor Bill Dalton, who along with three of the other four council leaders opposes the amalgamation proposal, said some Unison staff were "so offended" they sent him a copy of the letter.
"As you know, Unison is chaired by Kevin Atkinson, a leading light and founding member of ABHB," Mr Dalton said in the email to Hawke's Bay Today.
"More importantly, Unison is wholly owned by Hawke's Bay Power Consumers Trust - a trust belonging to the people of Hawke's Bay.
"For such an organisation to come out and effectively direct how their employees should vote is unacceptable and exactly the sort of action that could result in a challenge to the outcome of the poll."
Chief returning officer Warwick Lampp dismissed the complaint. He said that unlawful coercion must be specific to "the act" of voting - he used the analogy of someone standing over a voter at the polls.
He said it was similar to the situation of a complaint a few days after voting started on August 24, after campaign lobby A Better Hawke's Bay's founding, but now former, member Damon Harvey sent an email to business contacts urging them to make sure staff voted, preferably for a "positive change".
"There's no coercion," he said.
"It's just someone's opinion of where their view is headed.
"It's a piece of campaigning just like any other election."
Mr Lampp, who has had a 29-year career with local body elections, says most complaints usually relate to signage.
Offences including "corrupt practice" are covered by the Electoral Act along with other offences, with penalties ranging up to $50,000.
Additionally, any person is able to lodge a Petition for Inquiry into an election result if they believe particular illegal practices may have affected the outcome, but evidence would have to be produced before a District Court judge.
Mr Lampp started his association with local body elections as a returning officer at Papakura in 1986 and also had nine years' experience as administration manager of the Central Hawke's Bay District Council.
Mr Atkinson said last night he had not seen the letter, but had no concerns.
He said the power supply had seen "incredible benefits" from amalgamations over the years, from the days of power boards and the Napier Municipal Electricity Department.
But the network had "significant issues from time to time" in dealing with different councils and different by-laws," he said.
"The most important issue, however, is that we've got to get everyone to vote.
"Whichever way it goes, you want there to be a clear mandate, or else everyone would be left in limbo," he said.
Hawke's Bay Today was also told an amalgamation supporter was seen collecting voting papers from residents of a Havelock North retirement village and offering to fill them in on residents' behalf.
The claims highlighted growing tension as the three-week poll headed towards its deadline of Tuesday at midday, when election manager Electionz.com starts counting the votes to decide whether there should be a merger of the Hawke's Bay Regional Council and the Hastings, Napier, Central Hawke's Bay and Wairoa councils.
By late morning yesterday, 55,437 votes had been received, with a sharp reversal of trends since it was reported last week that voters in the Hastings District, the biggest of the council districts were lagging behind other areas in terms of the numbers of votes cast.
While it was reported on September 2 that Napier votes outnumbered Hastings votes by 1598, that margin had been slashed to 260 in yesterday's figures.
