Temuka farmer Jeremy Talbot captured this picture of the low level of Lake Opuah, near Fairlie, from a microlight aircraft yesterday.

A man-made lake supplying water to South Canterbury farmers could be empty for the first time in its 17-year history without significant rainfall in the next four weeks, its operators say.

Lake Opuha's reservoir was 25 per cent full and losing 0.7 per cent of its volume daily, Opuha Water Ltd (OWL) chief executive Tony McCormick said.

The farmer-owned dam near Fairlie had not received a significant rainfall since June and had never been drier in its 16 years.

"Looking at the moment, we could be looking at an empty lake by the end of February. It's obviously scary," McCormick said.

OWL's main job was to "keep the river going" but it also provided irrigation and some town water.

Farmers used that water for specialist crops and livestock feed, so doing without would have a "huge trickle" down for the wider community, McCormick said.

The Opuha Environmental Flow Release Advisory Group met on Tuesday to consider a response.

The group comprises district councils, Federated Farmers, Fish and Game, Forest and Bird, Arowhenua Runanga and Opuha Water. Its conclusions were "not particularly optimistic", McCormick said.

"This is the first time we've ever talked about an empty lake."

Timaru District Council (TDC) operations engineer Judy Blakemore said four of its water intakes were on restrictions - all linked to Opuha - and all 13 regionally were restricted in some way.

Domestic water users could use only a single hose, hand held or with a micro jet system, for a maximum of one hour daily between 7pm and 8am. Watering lawns was banned and businesses were asked to "minimise" their use.

TDC had not previously imposed anything more than a ban on watering lawns in the past five years. The last restrictions of any kind were in February 2013 "so it's really quite severe", Blakemore said.

McCormick said Opuha's plight had been compounded by a weak spring snow melt, which usually charged the dam before summer.

"We used up 25 per cent of storage in October. We typically fill that month but it went the other way this time."

The dam had always been 100 per cent reliable but it was now exposed to its inability to draw from the Southern Alps, McCormick said.

A strong northwesterly weather pattern was feeding alpine rivers like the nearby Rangitata but Opuha was mostly fed by easterly rain in the foothills.

The scheme's vulnerability to east coast dryness had always been on the cards, he said.

Environment Canterbury (ECan) monitoring and compliance team leader Jo Field "could not offer more praise" to OWL for its protection of flows.

The company and other groups had been working closely with ECan on staying within the consent limits.

"It's not in the best interests of consent holders to take total, total [allowed] takes because they're really just cutting off their nose to spite their face," Field said.