35 people were killed on the historic grounds of Port Arthur by Martin Bryant in 1996.
Victims have been remembered in Tasmania at the site where it all began.
35 tributes, for 35 lost lives.
Men, women and children were remembered by loved ones and by Australia's Prime Minister, Malcolm Turnbull.
"For those of you who lost loved ones and who witnessed the horror of what happened her we will never truly understand the burden you bear, the pain that you endure."
Around 500 people gathered, tearful as rain fell on the site where the shooting unfolded 20 years ago.
It was a grim milestone in a tragedy that became a catalyst for extraordinary change.
Opposition Leader Bill Shorten acknowledged the leader behind that change.
"But also out of that tragedy we produced the very best of the Australian spirit, we came together as a nation and from our unity we produced gun laws that have served Australians well ever since, for this we all owe (former Prime Minister) John Howard."
The shooting shocked the nation.
On April 28, 1996 at the Port Arthur Historical site, Martin Bryant walked into the Broad Arrow Cafe and fired on tourists, locals and workers with an assault rifle.
Bryant was given a life sentence for each person he killed.
His name was not spoken at the service.
Off-duty police officer Justin Noble was outside the cafe on the day of the shooting.
"He started moving the rifle around and trying to acquire targets and he was firing at people. And it's at that stage I said to my wife that we're in big trouble."
Maria Stacey was working at the site that day and has remained a staff member ever since.
She says it's a close-knit community.
"We deal with the massacre every day as staff members because visitors come here every day with an interest and lots of people, family members, 35 people killed and many, many others injured and many others here on the day, and you spread that out across the country there's an awful lot of people who have some connection."
The 10th anniversary memorial service in 2006 was meant to be the last.
There are many in Tasmania and across the country who say they would prefer to forget the mass shooting.
But others, deeply affected by it, say those who died need to be remembered.
Stephen Large, Chief Executive Officer of the Port Arthur Historic site, says the response from families of victims about a 20th memorial service was overwhelmingly in favour.
"It's really good to know there are some people that haven't been back to the site in 20 years that are coming to the service. And we certainly hope they get the same benefits that people coming back on the 10th anniversary in 2006 got from that particular service."
Tasman Mayor, Roseanne Heyward says the tragedy touched all Australians.
"Well it happened in this community and I don't think we can ever get over that it will always be remembered as part of our history. Although it happened in this community it had an effect all around Australia really."
