The world-changing events of September 11, 2001 happened 20 years ago, but Charles Sturt University psychology academics say there is a reason their effects are still so prominent.

Whether going to school, on the way to work, or just waking up, most people can recall where they were when they first heard the devastating news of the attacks in New York and Washington.

Charles Sturt University lecturer in the School of Psychology in Bathurst, Dr Nicole Sugden, says this is due to what is termed "flashbulb memory".

These are vivid memories and details of events, creating a snapshot of where you were and what you were doing when you heard this news.

Dr Sugden says most people have flashbulb memories for events such as 9/11 and the death of Princess Diana.

"These events tend to be surprising and challenge the way we see the world; for example, we don't expect planes to fly into buildings."

Dr Sugden says flashbulb memories become ingrained through the retelling of these stories in conversations or through the media.

Another theory is that these events lead to long-term memories because of the strong emotions they produce.

"People tend to be confident in their recall of flashbulb memories because they are so emotionally salient, however, studies have found the accuracy of flashbulb memories of 9/11 to decline over extended periods of time."

Dr Sugden says while memories might fade, the perceptions of people experiencing these events can remain distressing and shape future behaviours.

The world is reflecting on this tragedy and how it affected them and the world we live in, but few were as effected as those who were at ground zero during the tragedy.

Lecturer in the Charles Sturt University School of Psychology in Wagga Wagga, Associate Professor Gene Hodgins says the understanding of the effect of 9/11 on its victims has allowed for a better understanding of the broader effect of traumatic events.

Over the past 20 years, these effects have paved the way for creating better care and support for frontline and mental health workers.

Professor Hodgins says the 9/11 attacks on the United States have shown the negative impact on professionals who have cared for survivors through "vicarious traumatisation", which is the negative impact on a helper that results from empathic engagement with trauma survivors and their traumatic material.

"There is now much more recognition of the mental health impacts of being an emergency service provider.

"The wellbeing of police, fire, ambulance and other first responders is becoming more prominent, as are the important strategies needed to look after them."

Professor Hodgins says acknowledging the 20th anniversary of 9/11 shows the importance of anniversaries for those who have experienced stressful events. But they can also serve as a trigger for others who have experienced the trauma, especially firsthand.

"Individuals can have many personal triggers to their trauma, including smells, sounds and sights. The media need to be especially conscious of the vision, sound and descriptions they choose, and choose not to, display and the importance of pre-emptive trigger warnings."

Studies have shown some survivors of 9/11 have surpassed their pre-9/11 psychological functioning, indicating growing evidence for the existence of post-traumatic growth.

Professor Hodgins says: "While 9/11 was a traumatic event for many, and a pivotal moment in recent history, at least from the adversity we have been able to better understand the impacts that traumatic events can have on all of us."

Dr Nicole Sugden and Associate Professor Gene Hodgins are from Charles Sturt University.