Auckland's water shortage has been pushed to "critical" levels on the back of a new long-range forecast picking a dry second half of the year.

Watercare's dam levels were presently sitting at about 45 per cent, compared with the typical level of 78 per cent for this time of year.

The worst drought in a quarter of a century has deprived the region of about half its normal rainfall, forcing officials to put tough water restrictions in place.

MetService meteorologist Georgina Griffiths said long-range weather models have no good news for drought-stricken Auckland.

"The long-range models are sending a clear signal for a much drier-than-normal spring period, between August and November, with a strong ridge of high pressure preventing the rain makers from crossing Auckland very often," she said.

"This is the driest outlook for the spring season we've seen since 2013, when a major drought affected the upper North Island."

Watercare chief executive Raveen Jaduram says the forecast has moved Auckland's water supply status to "critical".

"We have been hoping for significant rainfall to replenish our dams, but the latest weather outlook indicates we're in for just the opposite – more dry weather for spring and early summer, in what has already been a record-breaking dry year," Jaduram said.

"But of course keeping our fingers crossed for rain is not all we've been doing. We are leaving no stone unturned in our quest to find more water sources or expand existing ones.

"Last week our board signed off on an urgent upgrade of the Onehunga Water Treatment Plant, which will deliver an extra four million litres of water a day (MLD) by September.

Watercare had already invoked emergency powers under the Resource Management Act to treat a further 15MLD from the Waikato River.

"This means we are now treating up to 165MLD at our Waikato treatment plant, and in August, when a new network reservoir is completed in Runciman, this will increase to 175MLD."

Work was also underway to bring back to service two former water sources – the Hays Creek Dam in Papakura and a bore in Pukekohe.

Site establishment works had already begun at both of these locations and building consent applications are being prepared.

The Pukekohe bore project is on track for delivery of an initial 2.5MLD in August, and a further 2.5MLD in December.

Modular, mobile treatment plants were being built inside shipping containers, while Hays Creek Dam would be supplying up to 6MLD by December.

These projects were expected to provide an additional 40MLD to Auckland, and would slow the storage decline of Auckland's nine dams in the Waitākere and Hūnua Ranges, which normally supply about two thirds of the city's water.

"To put that in perspective, that 40 million litres a day would supply a city the size of Dunedin," Jaduram said.

Desalination has also been investigated but was not a preferred option due to the cost and environmental impact.

Droughts like the current event are expected to become more frequent under climate change.