A spokesman for the Premier's department could not even guarantee the memorial would be built before the new station, which is set to be operational in seven years.

In the days that followed the December 2014 siege that claimed the lives of lawyer Katrina Dawson and cafe manager Tori Johnson, grieving Sydneysiders laid an impromptu memorial of flowers that filled a block of Martin Place.

Former premier Mike Baird promised survivors and families of victims in January 2015 a permanent memorial would be installed before the first anniversary of the siege by gunman Man Haron Monis.

"The outpouring of grief that was symbolised by a sea of flowers in Martin Place, and that moved hearts around the world, was the beginning of our recovery process," Mr Baird said in a statement announcing the memorial plans.

"The unveiling of a permanent memorial, on or before the first anniversary of the siege, will be another significant step in that process, and will guarantee that the memory of Tori and Katrina lives forever in the heart of Sydney."

But a year later, in December 2015, Mr Baird announced only the design of the permanent memorial, by Australian architect Richard Johnson, which would feature 400 flowers in lit boxes inlaid into the granite pavement in a "scattered starburst pattern".

Now, 16 months later, a spokesman from the Premier's department says "design work on the Martin Place memorial is progressing".

"The location of the memorial will take into consideration planned infrastructure projects at Martin Place, including Sydney Metro, and any existing services underground.

"The department will continue to speak with the families of Katrina Dawson and Tori Johnson regarding the next phase of the memorial design. The focus remains on creating a lasting memorial that will honour the memories of Tori and Katrina."

He would not give a date for its installation but said the memorial should be in place before the new station, which is due to be operational by 2024, "subject to final planning approvals and endorsement by all key stakeholders".

A spokeswoman for the City of Sydney said responsibility lay with the government. The memorial "is being managed by the Department of Premier and Cabinet," she said. "We are providing support to the state government with the development of the project."

The families of Ms Dawson and Mr Johnson could not be reached for comment.

Opposition Leader Luke Foley was critical of the delay and uncertainty around the project.

"Surely with all the resources of government, the Premier can tell us when and where the memorial will be installed," he said.

Construction of the Martin Place station for the new Sydney Metro rail line will involve the demolition of buildings between Castlereagh and Elizabeth Streets, and the creation of new underground connections to the existing Martin Place train station.

Macquarie Group wants to build two new commercial towers above the new station, a proposal that has proceeded to the third and final stage of the same unsolicited approvals process that delivered James Packer's Barangaroo casino to the city.

The coroner is expected to hand down his findings from the inquest into the deaths of Ms Dawson and Mr Johnson within weeks.