Orchardists and other water users in the Tasman district will have to reduce irrigation by 20% from Monday.
The district's dry weather task force has imposed the first stage of water rationing as river flows continue to drop.
A council spokesperson, Dennis Bush-King, says the critical period for irrigators, especially apple growers, is only weeks away.
He says at least 50 millimetres of rain is needed to avert further restrictions, but the long-range forecast is not promising.