A small commemoration service will be held at the Tasman Peninsula's historical site on Wednesday afternoon to remember those lost on April 28, 1996.
"We remember and send our love to all those who still bear the scars of that terrible day," the prime minister said in a statement.
"The families and friends of those who died, the injured, the survivors, the first responders and all those who witnessed and were impacted by the unspeakable horror of that day."
At the time, the incident was considered the world's worst mass shooting, with 35 people killed and 23 injured at the popular tourist site.
It remains Australia's most deadly massacre.
"We pay tribute to the strength, resilience and courage of the local Tasmanian community and to all those who could never forget but still found a way to heal," Mr Morrison said.
Tasmanian Premier Peter Gutwein said those who bore the emotional and physical toll would never be forgotten.
The shooting prompted significant gun reform under then-prime minister John Howard via the 1996 National Firearms Agreement.
The new law banned rapid-fire guns from civilian ownership except under certain, restricted licences.
It also tightened requirements for firearms licensing, registration and safe storage, and established a government buyback of semi-automatic and pump-action rifles and shotguns.
More than 650,000 weapons were destroyed. By some estimates, the move almost halved the number of gun-owning households.
"The country was united in horror and grief. And there was a very strong level of support for what we had to do," Mr Howard told the ABC.
Tasmania's former Labor premier Michael Field will deliver a welcome at the commemoration.
Torquil Canning, who designed the garden memorial that was built in 2000 and features a pool of peace, will also speak.
"It's still quite raw with a lot of people in the community. And certainly with myself, it deeply affects me," Tasman Council Mayor Kelly Spaulding told AAP.
"(As with) any tragedy in anybody's life, it doesn't get any easier necessarily, you just learn to cope with it."
Martin Bryant is serving 35 life sentences and more than a thousand additional years jail without parole after pleading guilty to the massacre.