Much of that water comes from the Hātea River while the Poroti Springs supplies between 30 per cent and 40 per cent of the city.
"Regardless of how good our systems are, there is no substitute for a lot of rain. We want the dam to be getting up to a good level by November which is traditionally our driest month, so we have good stores of water going into summer," Whangārei District Council water services' manager Andrew Venmore said.
"If that doesn't happen we will be sending out water saving messages by the end of this year. Although we can reduce the draw from the dam by using our full allocation of Hātea River water, the river's water level also falls in dry seasons."
The normal rainfall in Whangārei in May is about 135mm but just 80.2mm fell during last month, according to MetService.
Venmore said traditionally, this was the lowest the Whau Valley Dam could get to at this time of the year although it was down to 49 per cent at the beginning of winter last year, primarily due to drought.
Consistent rain in June and a storm in July filled up the dams last year, he said.
"The level will come up during winter but the concern is how much it will come up to before next summer. We need the dam to be above 80 per cent by the beginning of November.
"Heavy rain fills up the dam pretty quickly but not the aquifers but water tends to run off the surface."
Northland is likely to experience extended dry spells this winter that could affect river and dam levels heading into summer, Niwa's seasonal climate outlook for June to August has predicted.
There are currently no water restrictions in Northland, except in Dargaville and Baylys Beach where level two restrictions are in place.
According to MetService, Kerikeri recorded the highest rainfall in Northland at 99.6mm in the one month to Wednesday this week, Kaitaia 82mm, Kaikohe 68.4mm, Purerua 60.2mm, and Cape Reinga 53.8mm.
