Sydney is being smashed by a once-in-a-century swell as authorities warn lives could be lost in a massive coastal storm bringing waves up to 10 metres high.

An urgent warning has been issued as damaging gale force winds are forecast from Sydney to the Victorian border throughout the weekend.

WATCH THE VIDEO ABOVE: Dangerous coastal storm batters Sydney

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“Storm winds will exceed an average 100 kilometres an hour and so these winds on top of the swell will see our coast impacted heavily this weekend,” Surf Life Saving NSW’s Brent Manieri said.

“We had similar sized surf 12 months ago over the course of a weekend. We lost six lives on that weekend.”

The message from authorities is simple: “This weekend is a no-go zone on our coasts.”

Waves up to 10 metres high are forecast. Credit: 7NEWS

The severe weather has already caused major disruptions at Sydney Airport, which was cut down to one runway on Friday, forcing flight cancellations. Qantas alone cancelled 36 services, with airlines now scrambling to add extra flights for stranded passengers.

“My flight to Brisbane got cancelled. I’m gonna try and get a car and drive,” one teary-eyed traveller said.

Meanwhile, Friday night’s severe thunderstorm swept through with destructive force. Over 100,000 lightning strikes and two rainbows lit up Sydney’s sky, smashing the north and southwest the hardest.

“Places like Campbelltown did see more than 30 to 40 millimetres over a very short period of time. We did see those wind gusts around 50 to 60 kilometres an hour,” the Bureau of Meteorology’s Jonathon How said.

A lightning strike in Sydney on Friday night. Credit: 7NEWS

The winds whipped up dangerous conditions with trees crushing cars and power lines. The State Emergency Service responded to over 400 calls for help, dealing with flash flooding along Pittwater Road and destructive hail in Glen Alpine and Campbelltown.

Ferry services on Sydney Harbour will be cancelled from 8pm on Friday, with replacement buses brought in. Waves are expected to reach five metres at beaches like Bondi, and up to 10 metres offshore.

Authorities are warning residents to tie down loose items, avoid parking under trees, and never shelter under trees during storms. The hazardous surf conditions are expected to continue until at least Sunday afternoon.