Hamilton hit its highest ever recorded temperature on Tuesday, reaching 32.9C and beating the previous record of 32.6C set on March 11 2013.

The record-breaking temperature was announced by NIWA Weather as the ongoing heat wave sent Hamiltonians rushing to the river or nearest swimming pools, with some Southwell School students glad to be able to be able to get back to school and have a dip in the pool.

On Wednesday, the heat wave which has hit much of New Zealand subsided slightly with temperature reaching a high of 29C, according to MetService.

The heat, which will last until Saturday, meant an increase in water use and prompted Hamilton City Council to raise its water alert to level two.

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This means sprinklers and watering systems can only be used on alternate days — even street numbers on even days, odd street numbers on odd days — between 6am-8am and 6pm-8pm, while hand-held hosing can still be done any time.

On the council Facebook page, Linda Wallace commented that she was surprised that the city had lasted so long at level one throughout January, but noted a small flaw in the way the dates for sprinkling line up.

"Gutted though that I can't sprinkle for two days in a row this week, 31st and 1st. Almost seems discrimination. Worse on a leap year," Ms Wallace said.

It is not just water that Hamiltonians have been asked to keep an eye on, as the SPCA is urging all pet owners to be mindful of their pets in the heat.

SPCA CEO Andrea Midgen says pet owners need to be extra vigilant, with the sweltering temperatures this week capable of causing severe injuries to animals, even death.

"My key message for anyone going out with their pets this week is to consider: would you put yourself or your family through these conditions? If it's too hot for you to walk along the sand on the beach it will be too hot for your animals. If you can't bear to sit in the car for 10 minutes, neither can your animals."

Residents of Tauwhare, Gordonton and Matangi are also now on alert level two as their water is supplied by Hamilton City Council. Te Awamutu and Pirongia remain on alert level one.

City waters manager Maire Porter said a significant increase in water usage combined with consistently high temperatures has prompted the move to alert level two.

"Our residents have done a great job with water conservation so far this summer, but as people return from holiday, and with the warm settled weather we have seen a steady increase especially with outdoor water usage. With more warm weather on the way we need to ensure there is enough water for everyone," Ms Porter said.

Ms Porter says outdoor water use is the biggest contributor to increased water demand in summer.

"On average a sprinkler left on for an hour will use more than 1000 litres of water. Moving to alert level two and abiding by these simple watering rules will help conserve our city's water supply and give us a better chance of getting through the rest of the summer without the need for further restrictions."

Ms Porter says everyone can make small changes to their water use which can save a lot of water each day.

"Simple things like watering the garden by hand, taking shorter showers, not leaving the tap on when you're brushing your teeth or washing your car on a piece of lawn can also make a difference and will save hundreds of litres a day."