Wellington could face water rationing for the first time in living memory if a decent deluge doesn't fall in the next month.

Showers are forecast for tomorrow and Monday - but are not expected to make a significant impact on the dwindling water supply.

The dry spell has smashed records at weather stations in Wainuiomata and Kaitoke. At the Kelburn station, the rainless stretch is the longest recorded since 1919.

READ MORE: Heavy water users under microscope

MetService forecaster John Law said that after the rain cleared on Monday a cooler southwesterly flow would return and high pressure would build from Wednesday, bringing more dry weather through to next weekend.

Beyond that there was a lot of "variation and uncertainty" but the seasonal outlook showed normal autumn rain should return by April.

Wellington and Kapiti can expect 20 millimetres to 30mm of rain tomorrow and Monday. Wairarapa, Whanganui and Manawatu can expect 10mm to 20mm and Hawke's Bay and Gisborne 5mm to 15mm.

Rain in April is likely to replenish the rivers and stave off rationing, but with an unprecedented stretch of dry weather it was hard to know how long water would last, Greater Wellington Regional Council water supply manager Chris Laidlow said.

"It is a low probability but the longer we go with dry weather, the more we go into an area that we haven't been before. We are planning for a worst-case scenario."

An outdoor water ban comes into force in Wellington, the Hutt Valley and Porirua today and could help cut water use by 10 per cent.

But even then, significant rain is needed to avoid tapping into emergency water storage lakes at Te Marua, north of Upper Hutt, in the first week of April.

Dipping into the reserves could trigger even harsher water restrictions for the region, with the council reluctant to allow the emergency supply to fall below 30 per cent for health reasons.

"We need to preserve the storage in those lakes at all costs," Mr Laidlow said.

The regional council has been in discussions with local councils, Civil Defence and health authorities about how to respond if demand starts eating dangerously into the reserves.

"We have got advice from the Ministry of Health about the minimum volume of water needed for sanitary conditions."

Mr Laidlow confirmed rationing was one of the options but stressed it was unlikely, with substantial rain expected next month. "It would be something down the track."

He would not discuss details of the harsher options being considered. "We haven't agreed on what they would be yet."

The Wellington City Council, Civil Defence and Regional Public Health are participating in talks about rationing.

Lower Hutt Mayor Ray Wallace said talk of rationing was not helpful, with people already panicking about taps running dry.

"I don't believe it's getting that dire. We have plans for any contingency with this but we are not at the stage of needing to say ‘rationing'."

Early signs are encouraging, with only 140 million litres used on Thursday, 5 million litres down on the day before.

Mr Laidlow said water use needed to drop to 130 million litres a day to stretch into April.

REGIONAL WATER RESTRICTIONS

Councils around the lower North Island are monitoring water use and will tighten restrictions if the situation worsens.

Tararua District Council has imposed a total hosing ban for most of its district, with all hand-held hosing and sprinklers banned.

In Eketahuna, Woodville, Norsewood and Akitio, cleaning cars and washing houses or windows is banned, while in Pahiatua and Dannevirke, hoses and sprinklers may be used from 7-9pm on alternate days.

In South Wairarapa, sprinklers and irrigation devices for watering gardens are banned. But homeowners can use hand-held hoses on gardens on alternate days.

Masterton District Council has imposed an alternate-day sprinkler ban, but handheld hosing is not restricted.

Carterton District Council has recently fixed leaks to its new reservoir and has metering, thus avoiding restrictions.

Hastings residents, subject to limits for three months, have been asked to water their gardens from 6-8am and from 7-9pm. While they can use sprinklers during this time, residents in Whirinaki and Esk Valley cannot. Waimarama has a total ban.

Napier City Council has no formal water restrictions but urges conservation.

Palmerston North City Council has banned sprinklers, unattended hoses and garden irrigation. Hand-held hoses can be used only from 7-9pm on alternate days.

Ashhurst, Longburn and Bunnythorpe residents have no restrictions.

The Manawatu District Council has been trucking water from Feilding to supplement the Sanson supply. Sanson residents cannot use sprinklers and the council has asked them to avoid washing cars, paths and driveways. There were no other restrictions in the Manawatu district.

A Taupo council spokeswoman said it had ceased a substantial amount of its irrigation. It was using alternate water sources, instead of the town supply, to water some sports grounds. Other sports grounds were not being irrigated.