The warning comes as German authorities continue a massive manhunt for the culprit behind yesterday's Berlin truck attack that has left at least 12 people dead.
The self-proclaimed Islamic State has claimed responsibility for the Berlin truck attack, which left at least 12 people dead and wounded 45 more, many seriously.
IS claimed responsibility via its Aamaq Agency website, citing what it called a "security source," typical of how it claimed responsibility for previous attacks in Europe.
But the facts as well as the responsibility remain unclear for now after German police released the original suspect, a Pakistani national, from custody, citing a lack of evidence.
Authorities are warning people to remain vigilant and report suspicious movements, amid fears those behind the attack could strike again.
In Australia, Foreign Minister Julie Bishop says, while German authorities are only saying it is "assumed" to be a terrorist attack, the incident has all the hallmarks of such a plot.
Ms Bishop has told Channel Nine that Australians travelling overseas need to be cautious and have exit plans if they are in crowded places.
"Our travel advice for Germany was, in fact, updated on the 17th of December to warn people that Christmas markets and places of mass gatherings are vulnerable to attacks. And so we ask people to take care, to take the normal precautions, but, if they are visiting places like a Christmas market, to ensure that they have an exit plan worked out should there be a security incident of this type or worse."
Once again, people are gathering to mourn the dead and pay their respects to the victims of an attack aimed at nobody in particular.
The music was sombre as German chancellor Angela Merkel attended a church service in Berlin, then the site of the crash, in the aftermath.
Speaking at a briefing ahead of police officials, she presented her government's view on what may have transpired.
"The perpetrators will be punished as hard as our laws will demand. We will have to assume now that this is a terror attack. I know that it would be very difficult to bear for us that a human being has perpetrated this deed who has sought and found protection in Germany."
Before the Pakistani man was released, anti-immigration groups and a nationalist party were already blaming Ms Merkel's generous refugee policy for what happened.
Others, though, were simply taking the time to reflect on the immediate sadness.
Christian Sommer, a 21-year-old from Potsdam, near Berlin, attended a vigil at the attack site and says the pain goes far beyond Berlin and Germany.
"You can notice that Berlin is very sad, it is mourning, and I have to say that, overall, this is a very touching place. The whole thing is very distressing, especially if you see all the light, all the flowers. And it is a crushing blow for the entire city and the entirety of Europe."
As world leaders offered their condolences to Germany, the country's interior minister, Thomas de Maiziere, has acknowledged the true perpetrator may have escaped.
Berlin police chief Klaus Kandt says the search is continuing for those connected to the crash, and he says security, meanwhile, will be increased in Berlin.
"We will have stone barriers in various shapes to prevent this modus operandi to be repeated. We will have police with machine guns and with protective clothing."
Back in Australia, the official terrorism-threat level remains at "probable."
Julie Bishop has told the Nine Network 11 attacks were thwarted in Australia over the past two-and-a-half years.
"We have not changed our alert. It remains at probable. But we have ensured that our law-enforcement, security, intelligence agencies have the resources, the legislative power and the focus to ensure that we keep Australians as safe as possible."
