The state’s wet streak has caused Sydney’s major water supply, Warragamba Dam, to overflow for the second time this year.

The dam, which is four times the size of Sydney Harbour, began spilling over at 7.30am on Sunday.

“It was a slow build-up with lighter than expected rain over [Friday and Saturday] in the catchment area,” a WaterNSW spokesman said.

“We expect the spill to go for a few days at least, but it’s not a very large spill. It won’t be as large as the one last month, which peaked at 220 gigalitres a day. We were predicting this one would be 30 to 40 gigalitres a day, based on the lower end of the forecast.”

Warragamba, whose capacity is 2065 gigalitres, is located 30 minutes’ drive south-west of Penrith and supplies water to more than 5 million people in Sydney and its surrounds.

The dam was on Sunday spilling at a rate of 6.5 gigalitres per day and increasing, according to WaterNSW. It said other smaller dams in Greater Sydney were also continuing to exceed capacity, among them Cataract, Nepean, Woronora, Tallowa, Greaves Creek, Medlow Bath and Middle Cascades.

The overflow came as the Bureau of Meteorology issued a severe weather warning for the NSW South Coast, Southern Tablelands and Snowy Mountains, which was cancelled soon after 3pm.

The bureau forecast heavy rainfall on Sunday, leading to possible flash flooding, listing potentially affected areas as Nowra, Batemans Bay, Moruya Heads, Ulladulla, Narooma and Araluen, but the heavy rainfall had eased in the regions south of Sydney by mid-afternoon.