Christchurch City Council wants to change a resource consent so it can put wastewater into the Avon and Heathcote rivers more often.
The current consent requires the council to carry out work that would limit sewage overflows to once every two years, on average.
The council wants to change that to every six months on average.
Original estimates for the work were $40 million, and the work on the Heathcote River has been completed, the council says.
But further work is needed to meet the two yearly condition on the Avon River, which will cost $125 million.
Changing the consent to allow wastewater overflows every six months on average would allow more time to complete the work, moving the deadline to 2017.
Spokesperson Simon Collin said once finished the work is still unlikely to meet the condition limiting overflows to once every two years.
He said research has shown that more regular overflows will not harm aquatic life nor pose a health risk to people.