Southland's regional council is calling for a united response after the government announced councils have three months to come up with plans to amalgamate or it will be done for them.

The news comes as the Local Government Commission continues to investigate a proposal by Southland's mayor to merge the region's four existing councils into two unitary authorities.

The entire process from assessing the request to a final decision and transition was expected to take three to five years, but now the clock was ticking down with the government's new deadline.

Environment Southland chair Jeremy McPhail said the region had a limited window to determine its future.

"For Southland, the key question is whether we shape that change ourselves, or have it shaped for us," he said.

"It's up to us, as Southland councils, to work closely together to develop a proposal that reflects our region's unique communities, geography, and economy."

He was waiting for more government advice on how the Commission's investigation…