On April 28, 1996, 35 people were killed and 25 injured when a lone gunman opened fire with a semi-automatic rifle.
The tragedy, which remains Australia's worst modern mass shooting, prompted gun law reforms and a buyback scheme that destroyed more than 640,000 weapons.
The historical tourist site's memorial garden will on Tuesday hold an anniversary service from 1pm titled Words of Love.
A minute silence will take place at 1.30pm, the moment the gunman entered the Broad Arrow Cafe and killed 12 people.
Jane Scholefield, who hid behind a wall in the penitentiary building to avoid the shooting, will speak, as will a 17-year-old from the tight-knit Tasman Peninsula community.
A performance of Always Remember, a song written for the 20th anniversary, will be performed by local singers.
Media presence will be kept to a minimum - a decision made after consultation with survivors and family and friends of people who were killed.
Tasman Council deputy mayor Maria Stacey, who was at the site on April 28, 1996, said the event would be smaller than the 2016 service.
"We were always mindful of what it should be," she said.
"There have been some people in the past who are not comfortable with anything at all.
"But there are people who look to something being there to give them a place to go and be with people."
The demographic of the picturesque peninsula, which is home to a few thousand people, was changing with time, Ms Stacey said.
"(But) it's quietly there, the whole tragic event is always just under the surface," she said.
Some first responders who attended the scene, including some from interstate, are gathering at a different location to mark the solemn occasion.
Gunman Martin Bryant, who is now aged 58, is serving 35 life sentences and will never be released from Risdon Prison in Hobart.
Prime Minister Anthony Albanese thanked Walter Mikac, who lost his wife Nanette and daughters Alannah and Madeline in the shooting, for leading the call for gun reform in 1996.
"Australia is a better place because the government and the parliament of the day came together to answer Walter's call," he said in a statement.
"This is what we hold on to - the abiding memory that somehow amid the most terrible darkness the best of humanity found a way to shine."
The cruelty of that day remained beyond understanding, he said.
"We think of everyone whose world was shattered by the loss of those who had been the bright centre of their lives, their love left desperately wrapped around an absence," he said.
Gun reform has been under the recent microscope since 15 people were killed in a shooting targeting the Jewish community at Bondi in December.
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