Madeline Mikac would have been 23 this year, had she not been killed in the Port Arthur massacre.
The three-year-old, her six-year-old sister Alannah, and their mother Nanette were among 32 others who were fatally gunned down at the popular tourist site in 1996.
Thursday marks 20 years since the tragedy.
The Alannah & Madeline Foundation are holding a candlelight vigil at Federation Square on Thursday evening to commemorate the anniversary and remember the 35 people who were killed and the 23 who were injured.
The anniversary of the Port Arthur massacre also marks the many years of work by Alannah & Madeline Foundation to improve Australia's gun laws and protect children from violence.
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Walter Mikac launched the foundation with the help of friends and survivors 12 months after he lost his wife and daughters.
A number of Melbourne landmarks will be lit in purple, the foundation's colour, on Thursday to show their support.
Parliament House, the Melbourne Star, the State Library Victoria, Melbourne Town Hall, the Arts Centre spire, and AAMI Park are all set to turn purple as members of the community gather at Federation Square for the 6.30pm vigil.
