Auckland's water shortage will not get better in the next month, with tomorrow's heavy rain insufficient to replenish reservoirs after six months of historically low rainfall.
Total water storage in Auckland's dams is just below 60 per cent of capacity, well below the historic June average of 84 per cent.
Between 50-80mm of rain is likely to fall on Thursday in the Hūnua and Waitākere ranges dam catchment areas, Watercare spokeswoman Maxine Clayton said.
"This is the only significant rainfall forecast for the whole month of July. Typically we see somewhere in range of 200mm during July, so potentially we're looking for a rain deficit of 250-375mm since January.
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The January-June period was the driest on record for much of Auckland, Northland and the Waikato, according to the MetService. July was expected to be wet but no more than normal for this time of year - and not enough to make up the deficit.
Auckland's residents were asked on Tuesday to do their bit to save water and stave off the potential for water restrictions later in the year.
By tomorrow Watercare would be able to tell if demand had fallen, Clayton said.
Watercare was taking a "prudent" approach to managing the low rainfall, Clayton said. Customers would be told "well in advance" if restrictions were likely later in the year.
The last time mandatory water restrictions were imposed in Auckland was in 1993/94.
While Watercare's dams provide around 60 per cent of the city's water, it also has other resources, including the Waikato River and aquifers. There were fears that yesterday's massive sewage leak into Lake Taupō, which feeds the Waikato, would further threaten Auckland's water supply.
But Clayton said the Auckland Medical Officer of Health had confirmed it was safe to continue drawing water from the river.
"We are 400km from the leak. The wastewater will be fully diluted before it gets to our intake point in Tuakau, in a few days' time, and it will have no impact on our water supply," Clayton said.
Watercare's treatment plant would then treat the water to AA grade standard.
Watercare plan to draw more from Waikato River can't be fast-tracked
Auckland's application to get more water from the Waikato River can't be hurried along because the process is first-come, first served.
Watercare lodged an application in 2013 to boost its current limit of 150,000 cubic metres of water per day to 200,000.
Yesterday Mayor Phil Goff suggested that bid should be fast-tracked as prolonged dry weather began to bite.
But a Waikato Regional Council spokeswoman said with other applicants ahead in the queue, there was no timeframe for when the application would be processed.
If every application were approved, the river's water would be over-allocated, she said.
In 2004 it was predicted that Auckland would need the extra capacity from the Waikato River by 2021. But a push to improve water efficiency saw a big drop in consumption per person, and Watercare documents show the organisation now hopes to defer the need for extra river water until 2031.
5 top tips for being waterwise this winter
• Turn off the tap when brushing your teeth or shaving.
• Try to shower for four minutes or less.
• When hand-washing dishes, fill the sink rather than washing the dishes under a running tap.
• Wash full loads of laundry whenever possible. This will save water and energy.
• Check to make sure your toilet cistern isn't leaking.