"The poll allows Hawke's Bay people to have the final say on the best form of local government for them," the commission's chief executive, Sandra Preston, said yesterday.
"We encourage electors in the region to familiarise themselves with the commission's proposal so whichever way they vote they are making an informed decision."
Under the proposal, released in its final form on June 9, the Napier City Council, Wairoa, Hastings and Central Hawke's Bay District Councils, and the Hawke's Bay Regional Council would be replaced by a unitary Hawkes' Bay Council from November 1 next year.
If the proposal is supported, a transition body comprising two councillors from each existing council and an independent chairperson will be established to prepare for the election of the new council on October 8 next year.
Rangitikei district electoral officer Warwick Lampp has been appointed to conduct September's poll.
The date was selected to allow time to complete "statutory steps" such as preparation of the electoral roll, delivery of voting documents to electors, and a three-week postal voting period ahead of polling day.
Voting papers will be mailed out from Monday August 24.
The announcement of a poll date was welcomed by Rebecca Turner, chairwoman of pro-amalgamation group, A Better Hawke's Bay.
The group's initial 2013 proposal to the commission triggered this month's final proposal. "A Better Hawke's Bay has always believed that such a big decision should go to a referendum to ensure the new council has the mandate it will need.
"That is why we triggered the poll by getting affected Rangitikei residents to sign a petition last week," Ms Turner said.
"We will run a positive campaign explaining the benefits of amalgamation and how Hawke's Bay can move forward," she said.
Napier Mayor Bill Dalton, who opposes the commission's proposal, also welcomed confirmation of a date for the poll.
"Clearly I want one result rather than another, but the main thing I want is to get the amalgamation debate out of the way so we can get on with positive things about Hawke's Bay," he said.
Hastings Mayor Lawrence Yule said he was pleased the date that had been set meant there would be a "short, sharp" three-month campaign, rather than the debate being dragged out.