If Aucklanders cannot reduce their water savings by two buckets of water each, which is 20 litres a day, a boiling water notice would be issued.

That's the stark warning from Watercare as the city faces the largest water shortage in 23 years - 50 million litres a day needs to be saved.

A press conference was called to address the "critical need" for residents to reduce water use following massive rainfall in the region and more on its way. The rain caused unsettled silt in dams at a treatment plant where Auckland gets about half of its water from, meaning it was only able to treat half of what it normally did.

Spokesman Mark Bourne said if the savings aren't achieved, partially-treated water may have to be introduced into the system and a boiling water notice issued.

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The advice for residents is:

• No watering the garden
• No baths
• Three-minute shorter showers
• Avoid washing the car

Bourne held up a glass of untreated water at the press conference, which could have been mistaken for orange juice due to its colour.

Watercare's Joseph Challoner-Warman (left) and James Talbot looking at a water sample at the Hunua dams today. Photo / Supplied
Watercare's Joseph Challoner-Warman (left) and James Talbot looking at a water sample at the Hunua dams today. Photo / Supplied

Aucklanders on average use about 160 litres of water per person per day and about 450 million litres overall.

After a notice was put out yesterday to reduce usage, 420 million litres was used but the target was 400 million litres.

About half of Auckland's water comes from four dams in the Hunua Ranges, where flash flooding took place after heavy rain on Wednesday. Because of unsettled silt in the dams, the Ardmore Treatment Plant was treating about half of what it normally did.

"We're calling on Auckland ... to reduce their water use by 20 litres per person per day," Bourne said.

"Take a three-minute less shower, or don't take a bath. Avoid watering the garden and washing the car.

"We've had negligible rain fall last 48 hours but it's starting to rain again ... that's why it's imperative Auckland."

Bourne said the water supply network and infrastructure is designed to handle heavy rainfall. But when unprecedented events like this occur, it places additional strain on the treatment plants.

"While Ardmore is working at reduced capacity, our other treatment plants provide us with resilience to such events.

"This is unprecedented rainfall. Niwa says the extreme weather experienced this week is 'off the chart' and has well exceeded what would be considered a one in 100 year event."

He said the restrictions won't affect water pricing or Watercare's income.

The last time Auckland faced such a crisis was during the 1994 drought.

Watercare medical officer of health Dr David Sinclair said tap water was still safe to drink.

"There'll be more intense monitoring to see if anything is getting through. It gives assurance the water is safe.

"Normally the treatment plant makes water 10,000 times cleaner than it was, at the moment it's coming out 1000 cleaner which is still good quality drinking water."

Sinclair said chlorine kills off every bug and parasite except cryptosporidiosis which can cause diarrhoea and stomach pain. He explained that even if they introduced the partially-treated water there would be quite a low risk for illness and boiling the water was an effective preventative measure.

"[The bacteria] is at very low levels in the Hunuas anyway."

Sinclair's role was to ensure drinking water standards are being met.

He said the standard has currently been maintained.

Rain is battering the Auckland region and pelting down in Northland as Civil Defence warns of severe thunderstorms this afternoon and Upper North Island residents are also being warned of phone and internet outages.

The Coromandel and the Bay of Plenty are being warned to expect up to a month's worth of rain today - on top of the month's worth that has already fallen across the saturated regions this week.

Auckland Mayor Phil Goff told the Herald the water shortage was potentially serious.

"If we don't make water savings and we get a serious dump this afternoon and evening that could create real problems for the city," he said.

Goff was returning to Auckland from a meeting in Wellington with Prime Minister Bill English to visit Watercare's water treatment plant at Ardmore.

Watercare have maximised the production from the region's other four large water treatment plants, including Huia and Waikato.

A Watercare spokeswoman earlier said a long period of dry weather is needed for the water in the dams to settle so the Ardmore Water Treatment Plant can resume full activity.

Upper North Island residents are also being warned of phone and internet outages caused by extreme weather.

The top half of the North Island is seeing faults reported across the Chorus network.

In Whangamata, the core fibre damaged in a slip on Wednesday has been repaired, restoring broadband and voice services to customers of all but one of the Chorus roadside cabinets in the area.

Work is underway to repair a secondary fibre that will restore broadband and voice services to customers served by the final street cabinet. This work will complete today.

Chorus issued a statement this afternoon, apologising to customers who may have lost broadband or phone services. The company says it is working with its service companies to safely restore services as quickly as possible in affected regions.

Ways you can save water

• Cut your shower time by two minutes. Approx saving = 16L

• Turn off the tap when brushing your teeth. Approx saving = 4L

• Only run your dishwasher when full

• Only run washing machine when full

Today's weather:

Whangarei
Friday - Periods of rain, chance of heavy and thundery falls. Strong northeasterly.
High 25C, overnight low 20C

Saturday - Periods of rain, heavy at times. Northerlies.
High 27C, overnight low of 20C

Sunday - Periods of rain, chance of being heavy, but easing. Northwesterly.
High 27C, overnight low 19C

Auckland
Friday - Periods of rain, chance of heavy and thundery falls. Strong northeasterly.
High 25C, overnight low 19C

Saturday - Periods of rain, heavy at times. Northerlies.
High 26C, overnight low 20C

Sunday - Periods of rain, chance of being heavy, but easing. Northwesterlies.
High 25C, overnight low 19C

Hamilton
Friday - Rain at times, chance of heavy and thundery from afternoon. Northeasterly wind.
High 24C, overnight low 18C

Saturday - Rain, heavy falls, chance of thunderstorms. Northerly winds.
High 26C, overnight low 19C

Sunday - Periods of rain, chance heavy. Winds turn northwest.
High 25C, overnight low 17C

Tauranga
Friday - Periods of rain, chance of heavy and thundery falls.
High 23C, overnight low 20C

Saturday - Rain, heavy falls, chance of thunderstorm. Northerly winds.
High 25C, overnight low 20C

Sunday - Periods of rain, chance heavy. Winds turn northwest.
High 25C, overnight low 19C

Napier
Friday - Mostly cloudy, periods of rain from morning. Northeast.
High 23C, overnight low 18C

Saturday - Periods of rain. Northerlies.
High 26C, overnight low 18C

Sunday - Rain at times. Winds turning southerly.
High 26C, overnight low 18C

Wellington
Friday - Occasional rain from late morning. Light winds.
High 24C, overnight low 15C

Saturday - Periods of rain, chance heavy. Winds turn southerly.
High 19C, overnight low 14C

Sunday - Periods of rain, chance heavy. Strong southerly gale.
High 18, overnight low 14

Weather: what you need to know

• Heavy rain expected in Auckland between now and Sunday
• Take extreme care driving through heavy rain. Do not drive through floodwaters
• State Highway 25 at Whangamata remains closed because of a number of large slips
• Aucklanders are being asked to reduce their water use over the next three weeks after floods affected water treatment capacity
• Consider alternative plans if you have an outdoor event scheduled this weekend
• If you live on a rural property, think about your livestock rotation for the weekend, especially if your farm is flood-prone
• Check your drains and gutters to ensure these aren't blocked. These can cause flooding issues during heaving rain
• Tapapakanga Regional Park, Hunua Ranges Regional Park and Waitawa Regional Park have been closed because of heavy flooding