The Otago Regional Council is warning thousands of farmers with consent to take irrigation water from rivers to conserve as much as possible to reduce the pressure on rapidly dwindling rivers.

If farmers did not comply they risked being unable to take any water until rainfall replenished supply, council environmental services manager Martin King said yesterday.

The warning was not unexpected, he said, with Dunedin having had its driest December since records began in 1918. "When you think we get an average of 80mm in Dunedin in December and this year we only got 9mm, you can see why we are asking farmers to be careful."