Floral tributes for the Sydney siege victims will be removed and a marquee erected ahead of a forecast for rain.

Notes, messages and cards will be preserved when the existing blanket of flowers at Martin Place is taken down and composted on Tuesday.

Parts of the tribute for Lindt cafe manager Tori Johnson and barrister Katrina Dawson will remain under a marquee.

• Floral tribute to siege victims still growing

The removal of parts of the memorial has been designed to preserve the messages and not to discourage people from paying their respects at Martin Place, a spokeswoman for the NSW premier said.

An army of volunteers from the Red Cross, Rural Fire Service and State Emergency Service will help collect the flowers while chaplains remain to counsel visitors.

Condolence books will also be available at the Sydney Opera House and NSW Parliament House until the end of January.

They will be bound in several volumes and the Johnson and Dawson families will each receive a copy.

A permanent memorial will be discussed with the families once they are ready.

Financial donations in memory of the two victims have been directed to mental health organisation beyondblue and The Katrina Dawson Foundation.