The sea of flowers blanketing Sydney's Martin Place in tribute to the victims of the Lindt café siege will be composted and used to fertilise a future memorial site.

NSW government authorities making plans for the disposal of the floral bouquets are seeking advice from the city of London on how it went about composting the tens of thousands of flowers laid after Princess Diana's death in 1997, News Corp reports.

Authorities in Boston, which also dealt with a big public outpouring of grief following last year's marathon bombings, are also being consulted.

NSW Premier Mike Baird said: "The NSW Government is in contact with authorities in London and Boston regarding flowers, tributes notes and condolence books to ensure we are operating on prior international best practice."

He said the flowers will be composted and put at a memorial site yet to be determined.

Hostages Katrina Dawson, 34, and Tori Johnson, 38, died along with gunman Man Haron Monis in the early hours of Tuesday morning.

Prime Minister Tony Abbott and his wife Maggie Abbott arrive to pay their respects at the floral memorial in Martin Place. (9News)

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