A wayward sprinkler has flooded a major Sydney road, causing 30-kilometre traffic jams in the city’s south-west during Tuesday morning’s peak.

Overnight works in the M8 tunnel at Beverley Hills ran late on Tuesday after a fire sprinkler malfunctioned, flooding the tunnel, an NSW Transport Management Centre spokesperson said.

The tunnel was closed between St Peters Interchange and King Georges Rd at 9pm on Monday for routine roadworks and maintenance. It was meant to open at 5am, but was delayed due to the sprinkler issue.

One lane reopened in each direction on the M8 Motorway just before 8am. But even an hour later, traffic was still at a standstill, with a reduced speed limit of 40km/h because of water on the road.

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Thousands of drivers heading into the city on the M5 were caught in the delays.

Traffic was backed up more than 30 kilometres to Ingleburn in south-west Sydney, and remained heavy beyond 9am. Vehicles were also backed up on the M5 East at Bexley Road through to the M7 interchange at Prestons.

“That traffic is very heavy there on the M5 and the [alternative] surface roads are going to be very heavy as well,” the spokesperson said.

“The M5 is usually busy most mornings, to be honest. But this is worse than normal.”

Motorists were advised to plan for delays and use alternative routes.

““There’s gonna be a lot of angry people … it’s bumper to bumper … I’m going 15km/h and about to stop again,” one early commuter told Sydney’s 2GB radio.

Live Traffic NSW urged drivers to consider the M5 East Motorway between Beverley Hills and Arncliffe, as well as the M4-M8 Link between Haberfield/Wattle Street and St Peters.

Traffic was beginning to clear by mid-morning.