Former Australian prime minister John Howard is expected to pay tribute to victims of the September 11 Al-Qaeda attacks at a public remembrance service in Sydney.
In 2001, more than 2,750 people were killed when two passenger jets destroyed the Twin Towers, the symbol of New York's financial wealth and confidence.
Another jet slammed into the Pentagon, and a fourth jet crashed in a field in Pennsylvania after those on board tried to overpower the hijackers.
Today, the US ambassador and diplomatic representatives from Canada, Germany and India are due to join Mr Howard at St Andrew's Cathedral in Town Hall Square to commemorate the 15th anniversary of the attacks.
The service will be led by Dean of Sydney Kanishka Raffel.
Mr Howard, who was in Washington on official business when the Pentagon was hit, is expected to deliver a Bible reading.
The ceremony is due to begin at 10.30am.
Dr Keith Suter, economic and social commentator, believes Australia is at minimal risk of being a terror target, on a global scale.
“Of the total number if people killed in Islamic violence, five per cent have been in the Western world. So we are part of that very small five per cent," he told Sunrise on Sunday.
“The other 95 per cent are within the Islamic World, the Middle East, in Africa and the Sub Continent - that’s where the fighting is mostly carrying on. And we’re caught up in it but those countries are feeling it much more than we are.”
Obama urges unity
On the eve of the 15th anniversary of the September 11 terror attack, US President Barack Obama urged Americans to remain united in the face of terrorist attacks.
"In the face of terrorism, how we respond matters," Obama said in his weekly radio and online address.
"We cannot give in to those who would divide us. We cannot react in ways that erode the fabric of our society," he added.
"Because it's our diversity, our welcoming of all talent, our treating of everybody fairly, no matter their race, gender, ethnicity, or faith, that's part of what makes our country great. It's what makes us resilient," Obama said.
"And if we stay true to those values, we'll uphold the legacy of those we've lost, and keep our nation strong and free."
Evoking "one of the darkest [days] in our nation's history," Obama noted that much had changed over the past 15 years since the attacks.
"We delivered justice to [Al-Qaeda leader] Osama bin Laden. We've strengthened our homeland security. We've prevented attacks. We've saved lives," Obama said.
But at the same time, he said, referring to attacks in Boston, San Bernardino, and Orlando, Florida, "the terrorist threat has evolved."
"So in Afghanistan, Iraq, Syria and beyond, we'll stay relentless against terrorists like Al-Qaeda and [the Islamic State group] ISIL.
"We will destroy them. And we'll keep doing everything in our power to protect our homeland," Obama said.
New York remembers
In New York City, Americans mark the 15th anniversary more resilient, wealthier and more diverse than ever, yet that terrible day is forever seared into the city's DNA.
On September 11, 2001, the first foreign attack on the US mainland in nearly two centuries ruptured a sense of safety and plunged the West into wars still being fought today.
Nearly 75,000 others live with mental and physical illnesses as a result of the attacks, many of them emergency workers who breathed in cancerous toxins as they valiantly tried to save lives.
The World Trade Center site has been totally rebuilt, home to the National September 11 Memorial and Museum, the world's most expensive train station, a performing arts center and offices.
"People come to this site from all over the world. It is something that now is a symbol to people around the world of resilience," said Mayor Bill de Blasio.
The jewel in its crown is World Trade Center One, or Freedom Tower, which at 1,776 feet (the year of US independence), or 541 metres, is the tallest building in the Western hemisphere, its lights visible for miles.
Its observatory affords stunning views to rival the Empire State Building and the Oculus transport hub is an architectural marvel - a $4 billion glass and steel oval designed by Santiago Calatrava.
But New York remains on edge. Stringent security checks are routine. New Yorkers are taught to speak up if they see anything suspicious.
Elected officials speak constantly of making sure attacks never happen again, claiming to have thwarted 20 apparent terror plots.