A young Australian woman has cheated death for the third time this year after terrorist attacks took place in three different cities that she was travelling through.

Julia Monaco, 26, from Melbourne, was in a shopping mall with a friend when a van plowed into pedestrians in the Las Ramblas district of Barcelona on Thursday.

It was her third brush with terrorism since she began travelling around Europe three months ago but she told 3AW's Neil Mitchell that she won't let it stop her from seeing the world.

In June, she was put in a lockdown on the London Underground while out with friends when terrorists plowed into people on London Bridge before going on a knife rampage in nearby Borough Market.

Days later, she was in Notre Dame in Paris when a police officer was stabbed outside the famous cathedral.

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Julia Monaco was in a shopping mall with a friend when a van plowed into pedestrians in the Las Ramblas district of Barcelona on Thursday

'I don't feel like I want to go home,' she told Neil Mitchell on Friday morning.

'I feel like I want to stay here and not let them – whoever they may be – win.

'I'm going to see what I came here to see.'

Ms Monaco and her friends Alana Reader and Julia Rocca, also from Melbourne, were in the store on Placa de Catalyuna when they saw people outside running for their lives.

The doors were locked and she spoke of how she watched terrified pedestrians banging on the windows trying to get inside from the street.

'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.

It was her third brush with terrorism since she began travelling around Europe three months ago. Pictured from left, Julia Rocca, Steph Lamb, Julia Monaco and Alana Reader on holiday in Rome

Ms Monaco said those inside huddled at the back of an Urban Outfitters store and were told to lie face-down on the floor and away from the windows.

When they were finally allowed to leave, they had to walk back to their hostel and avoid the numerous streets that had been closed by police.

'You just have to keep going. I'm sure though tomorrow morning my mum will say 'come home', but I don't think I've been scared out of travelling,' she added.

Melbourne man Michael Christou was about 300m from the initial scene of the Las Ramblas carnage and was also nearby when eight people including two Australians were killed in the London Bridge van attack in June.

'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.'

Australian Adam James and his wife, who was pushed over and suffered minor injuries as they ran from the scene, were in Istanbul last year when a police bus was blown up in a terror attack.

People react and stand around in the Las Ramblas area in Barcelona, Spain as police investigate a damaged van, believed to be the one used in the terror attack

Foreign Minister Julie Bishop said 16 people were killed in the attack and at least 100 injured

'It's happened again. It's a very real thing,' he said.

Authorities said at least 13 people were killed in Thursday's attack - which has been claimed by Islamic State - and 100 more injured, 15 of them seriously.

Three Australians were injured, including an NSW woman who is in a serious but stable condition in hospital.

Prime Minister Malcolm Turnbull said Australia stood in 'absolute resolute solidarity' with Spain in the global battle against Islamist terrorism.

'This is a global battle against terrorism,' Mr Turnbull told reporters in Canberra on Friday.

Early Friday, Catalan police posted a tweet saying they shot and killed four suspects and wounded a fifth in a resort town south of Barcelona.

They said officers 'shot down the perpetrators' to 'respond to a terrorist attack.'

It wasn't immediately clear from the tweet if the five shot were suspects in the Las Ramblas attack or were allegedly targeting another location.

Foreign Minister Julie Bishop announced on Friday morning that one Australian woman is in hospital in a serious but stable condition

Las Ramblas, a street of stalls and shops that cuts through the center of Barcelona, is one of the city's top tourist destinations. People walk down a wide, pedestrian path in the center of the street while cars can travel on either side.

Police immediately cordoned off the city's broad avenue and ordered stores and nearby Metro and train stations to close.

Other witnesses also described horrific scenes and fearful crowds in the aftermath of the van attack, which has been claimed by the Islamic State.

MFB Commander Graeme O'Sullivan was one of the first responders at Melbourne's Bourke Street tragedy in January.

He and his wife saw the latest carnage unfold 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.

Forensic policemen arrive in the cordoned off area after a van plowed into the crowd

Spain has been on a security alert one step below the maximum since June 2015 following attacks elsewhere in Europe and Africa

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, also said the scene reminded her of the Bourke Street Mall rampage.

'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.

Keith Fleming, and American living in Barcelona, said he was watching television in his building on a side street just off Las Ramblas when he heard a noise and went out to the balcony to investigate.

He says he saw 'women and children just running and they looked terrified.'

Mr Fleming heard a bang, possibly from someone rolling down a store shutter, as more people raced by.

He said armed police arrived and pushed everyone a full block down the street.

Australian witnesses described the attack as reminiscent of the Bourke Strert Mall rampage in January (above)

State-owned broadcaster RTVE reported that investigators think two vans were used - one for the attack and a second as a getaway vehicle.

The attack in the northeastern Spanish city was the country's deadliest since 2004, when al-Qaeda-inspired bombers killed 192 people in coordinated attacks on Madrid's commuter trains.

Spain has been on a security alert one step below the maximum since June 2015 following attacks elsewhere in Europe and Africa.

Cars, trucks and vans have been the weapon of choice in multiple extremist attacks in Europe in the last year.

The most deadly was the driver of a tractor-trailer who targeted Bastille Day revelers in the southern French city of Nice in July 2016, killing 86 people.

In December 2016, 12 people died after a driver used a hijacked trick to drive into a Christmas market in Berlin.

There have been multiple attacks this year in London, where a man in a rented SUV plowed into pedestrians on Westminster Bridge, killing four people before he ran onto the grounds of Parliament and stabbed an unarmed police officer to death in March.

Four other men drove onto the sidewalk of London Bridge, unleashing a rampage with knives that killed eight people in June.

Another man also drove into pedestrians leaving a north London mosque later in June.