The memorial will also pay tribute to the police and emergency service officers who worked to end the 16-hour siege in mid-December.
The siege shocked Australians, with thousands of Sydneysiders laying bouquets
"The outpouring of grief that was
"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 (Johnson) and Katrina (Dawson) lives forever in the heart of Sydney."
Cafe manager Johnson, 34, barrister Dawson, 38, and gunman Man Haron Monis, 50, died in a bloody end to the siege. Several hostages were also injured.
News of the memorial, which will include compost created from the floral tributes, came as reports said Dawson was struck by police bullets when officers stormed the cafe.
Multiple police sources told The Sydney Morning Herald that the mother-of-three was not hit directly but possibly from a ricochet.
The Daily Telegraph added that a ricochet struck Dawson in the heart as she lay on the ground, with a separate ricochet striking her in the shoulder.
The newspaper, citing police sources, said Johnson was shot in the back of the head by Monis, who had 20 shotgun rounds in his pocket when he was killed by police.
NSW Police are conducting an investigation into the siege, with a spokesman telling AFP they would not comment on the newspaper reports.