UPDATE: Seven-year-old Australian boy missing after Barcelona terror attack.

UPDATE:  A seven-year-old boy who became separated from his mother is missing after the Barcelona terror attack, his Australian family say.

Sydney man Tony Cadman has posted on Facebook that his grandson Julian Cadman is missing and the boy’s mum Jom is in a serious but stable condition in hospital.

Foreign Minister Julie Bishop has said one Australian is missing and four others have been injured in the attack.

EARLIER: A NSW woman is in hospital and two Victorian men have been "affected" in a terrorist attack in the Spanish city of Barcelona that has been compared to Melbourne's Bourke Street Mall rampage.

Foreign Affairs Minister Julie Bishop said the woman was in a serious but stable condition. The exact condition of the men is unclear.

"They're in direct contact with consular staff. They have advised they were directly affected," Ms Bishop said in Melbourne on Friday.

"I'm not sure if they have injured or they just need to see a medical professional."

Consular officials were seeking more information on the injured NSW woman. Officials are also scouring hospitals in search of any other Australians.

"We know she is in a serious but stable condition. Our consular staff will be at the hospital as soon as possible."

More than 13 people were killed - Ms Bishop put the toll at 16 - and more than 100 injured when a white van ploughed into crowds on Las Ramblas.

Witnesses described horrific scenes and fearful crowds as the attack, which has been claimed by Islamic State, took place.

Prime Minister Malcolm Turnbull said Australia stood in solidarity with of Spain in the fight against IS terrorism.

"This is a global battle," he said in Canberra.

Ms Bishop described the attack as an "atrocity".

"The Australian government condemns this brutal and deliberate act clearly designed to harm tourists on holidays in Spain," she said.

Australian cyber safety expert Susan McLean was about 100m away as the van zigzagged down the busy avenue, mowing down pedestrians and leaving bodies strewn across the ground.

"All of a sudden there was this tidal wave of people running from both Placa de Catalunya and Las Ramblas towards us screaming, crying and with absolute terror etched on their faces," she told Nine Network on Friday.

"Several of them were calling 'gun, gun', so first of all we thought someone had been shot.

"Then they just kept sort of - it was all in Spanish, it was very difficult to understand - but they were sort of pushing us into shops."

Ms McLean, who was separated from her husband in the panic, said the scene reminded her of Melbourne's January 20 Bourke Street Mall rampage in which six people were killed and dozens more injured.

"My first reaction was the Bourke Street massacre, because that is what it reminded me of - the vision of people fleeing in just such terror," she said.

MFB Commander Graeme O'Sullivan was one of the first responders at Melbourne's Bourke Street tragedy. He and his wife saw the latest carnage from the rooftop of their Barcelona hotel.

"We were up on the sixth floor roof terrace, just the pool area, enjoying a few drinks," he told Nine.

"We could clearly hear thuds as the vehicle was running into people, and then a short time after that, obviously, several very loud sickening screams from the people involved down at street level."

Mr O'Sullivan said the similarity to the Bourke Street Mall event was chilling.

"Bourke Street wasn't terrorism and this appears to be, but the result is still the same," he told Melbourne radio 3AW.

"It's a terrible scene here."

Australian Gil Van Der Venne also witnessed the attack unfold from his hotel balcony.

"You hear this screaming, the mayhem," he told 3AW.

"There was at least five bodies that I saw.

"The van, I believe, had probably travelled about three-quarters down Las Ramblas."

Melbourne holiday-maker Julia Monaco said local police ordered her and her family inside a shop as as the attack happened.

"In a split second it all kind of changed and everyone just started running and panicking and running for their lives and crying and screaming and we were forced back into the store, told to get away from the windows and to get low on the ground," she told Nine.

"We were huddled at the very back of the store, lying flat on the floor, and were in that position for about 20 minutes."

Michael Christou, also from Melbourne, was with friends about 300m away.

"We decided to get out of there. Everyone started to close the door shops so we just ran inside the shop and they closed the door."

Chillingly, this is not Mr Christou's first experience of terrorism.

He was also nearby when the London Bridge van attack happened on June 3, killing eight people, including two Australians, and injuring 48.

"I think it's following me but you kind of come over here (to Europe) and you expect it to happen but you don't let it stop you from doing what you want to do."

Australia's national terrorism threat level remains at probable.