A three year investigation into Christchurch's water supply has found local authorities do not know how much water will be needed as the city grows.
It comes at a time when the city's aquifers have hit record lows, and the councils are looking at ways to cut the amount of water being used.
A report on the situation by Golder Associates was commissioned by the city council, and cost $20,000.
It said a model needed to be developed to calculate the "sustainable yield" of the city water supply, based on population growth, the rural water demand and industrial use.
It also said more information was needed about how the water supply could be contaminated, and agencies responsible for managing it needed to work together better.
The recommendations are set to be put to a city council committee tonight, which will then discuss what the next steps should be.
Former Environment Canterbury chief executive and environmental planner Bryan Jenkins said the city council and regional council had been working together four years ago to estimate how much water could sustainably be taken from the aquifers.
But he said that project had been put on hold as staff focused on other priorities.
Per person, he said Cantabrians used a lot more water than in other cities, so he said something would need to be done to reduce the amount of water used.
That could mean charging Cantabrians for the water they used, he said.
The report is the latest in a series on the state of the aquifers, which the city council began in 2013.
It commissioned Golder Associates to look into how much was known about the aquifers, and what the gaps in knowledge were.
As part of the work it has talked to experts from the different agencies responsible for the aquifers, and brought them together for a workshop.
It now recommends another workshop should be held between city council, ECan and Selwyn District Council staff to look at who is responsible for what and how they communicate.
The report said different agencies managing the water had not always communicated with one another.
That meant some work done on the water was redundant, as other agencies had done very similar work but had not let others know.
It said the biggest issue was between ECan and the city council, but it also affected Selwyn District Council, academics and Government scientists.
It said working together would be crucial if they were to improve the protection and management of the water resources.
"A fundamentally different way of organising and communicating the value of Christchurch's groundwater resource and how best to protect and manage this asset is needed."
City councillor Pauline Cotter, who sits on the committee looking at the issue, said communication was an issue because of the number of different agencies involved.
She said the city council was already planning a "summit" to encourage more collaboration.
She said more work would need to be done to gather information about the water supply before they could make decisions about the best way to protect it.
Where water from Christchurch aquifers goes:
• 45% Residential and commercial
• 31% Industrial
• 22% Rural or irrigation
• 1% Recreational
By the numbers:
• Christchurch residents use an average of 357 litres of water per person per day
• It costs the city council about 45 cents per household per day to supply water
• 48 million cubic metres of water on average is used in Christchurch each year