A Christchurch City Council staff report has put the cost of a tidal barrier on the Avon and Heathcote Rivers at $430 million, rising to $500 million if the sea rises one metre due to climate change.
The barrier is being looked at as a way to prevent flooding in low-lying areas. It investigates what would be needed if the climate stays the same, and what would be required if the sea level rises in coming years.
It has also considered alternative solutions such as purchasing residential red zone land and building stopbanks alongside the Avon-Heathcote Estuary and the Avon River.
If sea levels stay the same, the cost of doing this is put at $211 million.
However, if sea levels rise by a metre, the report says the cost of the stopbanks would climb dramatically to $545 million.
It said the Canterbury Earthquake Recovery Authority had advised any decision on building stopbanks would be a Cabinet decision.
The report goes on to say the cost of the barriers would be prohibitive and recommends the council does further work with the recovery authority to come up with a solution.
The report will be considered by a council committee next Thursday.