A café in Sydney that was the site of a terror attack three months ago reopened on Friday.

Hundreds of customers lined up outside the Lindt Chocolat Café to show their support for the business, and an employee who was held hostage for 16 hours was among those working.

"You've got to come here united," said Praveen Swarup, a customer who was first in line. "This is the best way to overcome an incident like what we had."

On December 15, Iranian refugee Man Haron Monis held the café's employees and customers hostage at gunpoint as he demanded to speak to Prime Minister Tony Abbott, receive an Islamic State flag, and pass a message to his "brothers" not to detonate two bombs in the city. Officials later learned that the bomb threat was a hoax.

Lindt manager Tori Johnson was killed by Monis before police stormed in nearly a day later, and customer Katrina Dawson died after being shot in the ensuing gunfight. The reopened café features two plaques honouring the fallen victims.

Nine customers and seven employees escaped with their lives, and Monis was killed by police.

Escaped hostage and café employee Joel Herat said he found it necessary to return to work last week.

"It was extremely important for me to be here and support Lindt and support the people I work with," he told the Australian Broadcasting Corp. "Working with these people here, it's helped a lot in the healing process."

Lindt CEO Steve Loane and New South Wales state Premier Mike Baird agreed that reopening the business was the right decision.

"It's a difficult day, but in many respects a hopeful day," Baird explained. "The city was tested, it was challenged, but today is a strong reminder that we march forward."