Local Government Minister Simon Watts and RMA Reform Minister Chris Bishop announced the move on Tuesday afternoon, giving a three-month deadline for reorganisation plans to be delivered.
It followed an announcement in November that groups of city and district mayors — with some government oversight — would be formed to come up with such plans.
But the ministers on Tuesday said if councils failed to make use of the new "Head Start" approach, they would be forced into changes.
"Our message to councils is simple: lead your own reform, or we will do it for you.
"Either way, change is coming," Mr Bishop said.
Mr Watts was explicit that proposals should focus on creating larger, more efficient unitary authorities.
He said the government was providing councils with "real flexibility".
"Proposals don’t need to cover an entire region, but they must show clear support, strong leadership and real benefits for communities."
The proposals would be considered by government officials against criteria including practicality, simplicity, value for money, effective representation, timeliness and how it works with the new resource management system.
"For areas that do not come forward through the head start pathway, the government will implement a backstop process to ensure reform still happens across the country.
"This will involve a standardised approach, including transitional governance arrangements while changes are put in place," Mr Watts said.
In a statement Timaru District Mayor Nigel Bowen voiced his support for the announcement.
"We welcome the opportunity to become a larger unitary council and take greater control of our future.
"For too long, the urban voice of Christchurch has dominated regional policy, often failing to reflect the priorities and perspectives of South Canterbury urban and rural communities.
"This is our opportunity to reset that balance and shape a model that works for town, country and every place in between."
He said he would begin actively engaging with neighbouring councils to progress options for a new local government structure across South Canterbury.
"As part of this, we will also engage directly with Environment Canterbury to assess how regional functions could be better aligned or integrated within a future unitary structure.
"We recognise and support the importance of progressing this work ahead of the 2028 local government elections, so that the future structure is clearly defined and communities can have confidence in how they are represented before they are asked to vote." — RNZ/Allied Media
