NERVOUS Sydneysiders thought the worst when they were rocked awake in the early hours by a mystery — and extremely loud — bang.

Steph, from Sydney’s lower North Shore told news.com.au “I genuinely thought the Harbour Bridge had blown up it was so loud.

“The apartment building shook and the blinds fell down in our lounge room.

“My sister lives a block away and texted me to ask if I’d heard an explosion too.”

People on Twitter were equally nervous.

“Heard a massive explosion from Sydney Harbour. What is going on?” one Tweet, reflective of many, said.

“What the hell was that sound over Sydney?” said another. “Wtf was that? It sounded like an explosion right next to me.”

Others wondered if it could have been a plane crash or, in the wake of Monday’s attack in Manchester, an altogether more sinister event.

ABC News TV presenter Juanita Phillips even took to twitter to tell Sydney she’d driven down to the Harbour Bridge, “with the intention of helping the injured”.

“We honestly thought a bomb had taken out the bridge,” she said.

Another social media user asked the Twittersphere, “So was the HUGE bang at 5am just thunder? I thought someone just nuked us.”

@sarahvmac We honestly thought a bomb had taken out the bridge. Went there with the intention of helping injured. What a world we live in. — Juanita Phillips (@Juanita_Phillip) May 23, 2017

Heard a giant explosion coming from Sydney harbor. What is going on? — shelter (@Sh3lter) May 23, 2017

Sydney loud noise heard, folks trying to figure out if it was explosion, sonic boom or what. — Zr1Trader (@ZR1Trader) May 23, 2017

What the hell was that sound over Sydney? — David Alexander (@davidFalexander) May 23, 2017

Well, we can indeed confirm it was just thunder. But even the Bureau of Meteorology weather experts agreed it sounded louder than usual.

Meteorologist Steph Spackman was travelling to work at 4.30am when the thunder jolted her. “I was at Epping when I heard a huge thunderclap overhead,” she told news.com.au.

The lightning cracks appeared to be particularly loud on Sydney’s north shore.

“Everyone loves thunder,” Ms Spackman said. “Just not at 5am in the morning before they get up.”

Sydney’s rather noisy start to the day comes as Brisbane, yet again, struggles to get to work through thick fog delaying flights at the city’s airport.

Ms Spackman said a low pressure trough crossed parts of NSW early on Wednesday morning and hit a southerly change that came up the coast.

Combined they created the perfect conditions for an ear splitting early morning weather event.

Lightning radars recorded more than 4500 strikes in the wee hours.

Morning! as at 06:30, services are on or close to time.

So did anyone else need to grab their Thunder Buddy this morning?

This was us... pic.twitter.com/LKMtWG63nC — T4 Sydney Trains (@T4SydneyTrains) May 23, 2017

The loudest thunder strike I've ever heard in North Sydney just happened, definitely need a new pair of pants after that pic.twitter.com/ldhUvEWvID — John Loy 💭 (@johnwloy) May 23, 2017

It wasn’t just Sydney either. Storms hit the south coast, Canberra, Blue Mountains, the Hunter, North West slopes and Tablelands.

But what about the volume of the thunderclaps?

Morning commuters told news.com.au they were “louder than any thunder than I’ve ever heard” and the sound was “terrifying”.

Ms Spackman has a few theories as to why it probably seemed worse than it actually was.

Firstly, many thunderstorms close to Sydney are actually centred offshore meaning the sound weakens as it travels towards the eardrums of the city’s citizens. In contrast, this one passed directly overhead.

“It was just a normal thunderstorm but I think people thought it was louder because it was the dead of night, everyone was asleep.

“You don’t have the usual background hum of the city at 3am so there’s nothing to compete with the sound of the thunderstorm,” said Ms Spackman.

Most of the storms have now headed out to sea leading to what could be a sunny afternoon across Sydney with a high of 23C.

FOG IN BRISBANE, AGAIN!

Up in Brisbane it’s fog not thunder causing issues. For the third day in the row, the city has been blanketed by a thick blanket.

Visibility has been reduced to about 300 metres at the airport and least two dozen flights in and out of the river city have been delayed, said AAP.

The fog is thickest along the Brisbane River but conditions are expected to ease quickly, authorities said.

Some CityCat services were delayed earlier in the morning, but are back on schedule.

A high of 27C can be expected when the fog clears in Brisbane.

Further south, Melbourne will see a high of 17C and showers, Adelaide will get to 19C but, again, with showers.

The rain theme continues in Perth with the mercury rising to 20C while Hobart and Canberra are on 15C.

benedict.brook@news.com.au