Fifteen years on from the terrorist attack, John Braithwaite still has vivid memories of that day – he saw the first plane hit from his New York office.

"I saw the first plane fly up Fifth Avenue from my office window. There was all this noise outside and people screaming," he said.

"I was then standing out on the street when the second plane hit."

The Canberra resident and academic was in the US at the time as a research appointment at New York University.

Like many who witnessed the attack, Mr Braithwaite said the day started just like any other, preparing for classes that would never happen.

"People were standing in the street after the first plane hit saying it was amazing accident. After the second plane hit, people realised it could not be an accident," he said.

"A lot of people were listening to the radio on the street and they made an announcement saying there were fears that the buildings could collapse. At that point I locked myself in my unit."

Mr Braithwaite said he also closed all the windows as he had initial fears the hijacked plane also contained poisonous gas.

Canberra friends Christina Sparrow and Sarah Hill were also in New York on that fateful day.

The pair were on a holiday and were planning to spend a week in New York, having flown in from Paris on September 10.

"When we heard the plane fly into the building, we thought it might have been a Cessna or something, we didn't realise it was a 737, and that was frightening," Mrs Sparrow said.

"If we hadn't have slept in because we were jet-lagged, we would have been walking past the World Trade Centre at the time of the attacks."

In the hours following the incident, Mrs Hill said there was much confusion on the street.

"There were people wandering around in a daze, many didn't realise what was happening at the time," she said.

"What would normally be busy streets were empty. Cabs pulled over to the side of the road with the radio on. Everyone was just trying to find out what was going on."

The friends made their way to the Australian Consulate, where they were able to contact family back home.

Following the terrorist attack which killed 2996 people, Mr Braithwaite said the mood around New York wasn't an angry one, instead it was one of reflection.

"You think of New York as a city full of aggressive people, but in the week afterwards people were reflective and upset about it, wondering how anybody could have done something like this," he said.

"Very quickly the streets were full of signs of people saying they were looking for their family members."

Both Mrs Sparrow and Mrs Hill abandoned the rest of the New York trip, heading across the border into Canada.

They said they couldn't stay to go sightseeing when New York was still in mourning.

"How could you stay in a city that had gone through a situation like that and enjoy yourself? It would have been pretty disrespectful," Mrs Sparrow said.

"While we were happy that we were both safe, we really felt for the locals," Mrs Hill said.