"When we get back-to-back days of high temperatures, we struggle to get water through the network fast enough to meet peak demand.
"That means there's a risk that we can't keep supplying drinking water at a constant flow and pressure, which can create problems in the network. There's also the risk that we won't have enough water in reserve to fight fires if we need to."
Under the restrictions, residents at odd-numbered addresses can water their gardens before 3pm or after 9pm on odd dates and residents at even-numbered addresses can water before 3pm or after 9pm on even dates.
Beaumont said the council is keeping a close watch on the weather and the city's water use.
"We are monitoring the weather forecasts and the city's water use on a daily basis. If we need to, we'll move to a higher level of restrictions."
Level 2 water restrictions were introduced last month in Akaroa, Duvauchelle and Takamatua on Banks Peninsula.