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.