Mid Canterbury farmers should be checking their well levels to make sure Saturday's big quake has not changed underground aquifers.

Environment Canterbury acting director for investigations and monitoring Tim Davie said the 7.1 magnitude earthquake could have induced a response in groundwater up to several hundred kilometres from its epicentre.

"In the long term, we are not expecting any major changes. Mid Canterbury farmers might well have experienced water level rises if they are looking at their wells, or even some drops."

He said the fluctuations were a normal response to the quake and would settle down within a few weeks.

Across the Rakaia, wells have shown marked increases in water level and spring-fed streams such as the Halswell River have also risen.

Increased turbidity (cloudiness) had also been seen in some wells, said Dr Davie.

In the extreme event of a well drying up, ECan is allowing farmers to bypass the usual consent process and redrill immediately.

The changes are due to:

• uplift occurring along one side of a fault line which will raise the water level

• the compression of aquifer material which effectively squeezes water out of the ground

• the generation of a pressure wave through the groundwater that will raise water levels in places, and

• the reconfiguration of an aquifer through the shaking of its material.

"Of these, only the last mechanism will produce permanent changes.

In the past four days scientists from GNS Science and ECan have been measuring some of the geological changes and the differences in water levels and streams response. This will continue over the next 7-10 days. In particular a large-scale monitoring of groundwater bores will be carried out that will give a better idea of how much of the change is permanent.

"We will release the information from these observations as soon as possible to alert water users to potential risks."

Dr Davie said scientists had been surprised at how well levels had risen. "Although we knew there would be a response, some wells have changed a lot. Some have gone up tens of metres and springs have popped up in places not seen before."

Farmers may also need to consider damage to well casings and screens. IrrigationNZ has information on how well owners can check for this type of damage. Visit their website http://irrigationnz.co.nz/