Sydney barrister Katrina Dawson, 38, and Lindt Cafe manager Tori Johnson, 34, were killed during the 16-hour siege carried out by 50-year-old gunman Man Haron Monis.

Former NSW premier Mike Baird promised the memorial in January 2015 but its installation was stalled by infrastructure works in Martin Place for the new Sydney Metro Station.

The memorial was designed by Richard Johnson and artist Jess Dare, and was inspired by the mounds of flowers left by mourners who flooded Martin Place in the immediate aftermath of the siege.

NSW Premier Gladys Berejiklian unveiled the tribute on Saturday, the third anniversary of the siege, dedicating it as a place of reflection for generations of Australians to come.

"We gather to dedicate a permanent memorial to the victims of the Martin Place siege to ensure that Australians today and for generations to come will always understand and reflect on what took place here," she said.

"On the ground before us, more than 200 mirrored boxes have been embedded, each containing an individual handcrafted flower."

"During the day these flowers will reflect the sun, at night they will be lit from above, illuminating Martin Place."

At the centrepiece of the memorial two clusters of aqua hydrangeas and yellow sunflowers were laid, Ms Dawson's and Mr Johnson's respective favourites.

"The design concept creates a quiet, reflective and symbolic memory of the floral tributes left in the days following the siege, while working with the functional surroundings of the very busy pedestrian area that Martin Place is," Professor Johnson told Fairfax Media last year.

The families of Ms Dawson and Mr Johnson were present and participated in the ceremony to commemorate their loved ones.

"Like the original floral tribute, this memorial draws strength from its beauty and simplicity," Ms Berejiklian said.

"It says more than we could do with words alone. It is titled Reflection and it will indeed be a place to reflect."