Usually it's the articles from 2016 that have been quoted, cited, meme'd, etc.... The Gaurdian and NBC were the ones pushing the story that year, and everyone jumped on it. When you go to Wikipedia's section for massacres, they count all 16 involving firearms. But when you dig deeper you see that two of the massacres weren't mass shootings. The Top End Shootings in 1987 happened over 5 days. 3 people shot in one instance and five days alter two more. The Cangai Siege happened over 9 days. They use Wikipedia as a quick reference, and at first glance it backs up what they say.

Here are the updated lists for Mass murders, I found more before & afters. 22 before 1996, and 24 after.

Australia Mass Murders before 1996 National Firearms Agreement

Savoy Hotel fire, 1975 15 Dead, 25 injured by arson Connellan Air Disaster, 1976 5 Dead from deliberately crashing plane Wolvi Farm Horror, 1977 4 Dead. 1 by firearm, 3 by bludgeoning Campsie Murders, 1981 5 Dead by Firearm Pacific Nursing Home fire, 1981 16 Dead by arson
Inland Motel murders, 1983 5 Dead, 16 injured by vehicle Wahroonga murders, 1984 5 Dead by firearm Millperra Massacre, 1984 7 Dead, 28 wounded by firearm Oliver Murders, 1986 4 Dead, 1 wounded. 3 dead by bludgeoning , 1 Dead & 1 wounded by firearm, Hoddle Street Massacre, 1987 7 Dead, 19 wounded by firearm Canley Vale Huynh family murders, 1987 5 Dead, 1 wounded by firearm Queen Street massacre, 1987 9 Dead, 5 injured by firearm Richard Maddrell Murders, 1987 4 Dead by firearm Oenpelli shootings, 1988 5 Dead by firearm Downunder Hostel Fire, 1989 6 Dead, 16 injured by arson Edward Snow St. George Murders, 1990 4 Dead. 2 dead by firearm & 2 Dead from throats cut
Surry Hills shootings, 1990 5 Dead, 7 wounded by firearm Strathfield Massacre, 1991 8 Dead, 6 wounded by firearm Central Coast Massacre, 1992 7 Dead, 1 wounded by firearm Greenough Family Massacre, 1993 4 Dead by Axe Hillcrest murders, 1996 5 Dead by firearm Port Arthur Massacre, 1996 35 Dead, 24 Wounded by firearm

Australia Mass Murders Since 1996 National Firearms Agreement

Peter Shoobridge the Tasmanian Devil murders, 1997 4 Dead by knife, 1 dead by firearm Ronald Jonker Family Murders, 1998 4 dead by Car Exhaust Gas Barbara-Anne Wyrzykowski Family Murders, 1999 6 Dead by Car Exhaust Gas Mark Andrew Heath Family Murders, 1999 5 Dead by Car Exhaust Gas Childers Palace Backpackers hostel Fire, 2000 15 Dead by Arson Phithak Kongsom Murders, 2003 4 Dead by stabbing Michael Richardson Murders, 2004 4 Dead. 2 by suffocation, 1 stabbing, 1 firearm Oakhampton Heights triple-murder suicide, 2005 4 Dead by firearm Gary Mark Bell Murders, 2008 4 Dead by gassing Churchill-Jeeralang Blaze, 2009 21 Dead from Arson Lin Family Murders, 2009 5 Dead by Bludgeoning Roxburgh Park Osbourne murders, 2010 4 Dead by firearm Paul Rogers Murders, 2011 4 Dead. 2 by stabbing & 2 by gassing Quakers Hill Nursing Home Fire, 2011 14 Dead, 5 injured by Arson Gold Coast Quadruple Homicide, 2011 4 Dead by stabbing and asphyxiation Sharma Family Glen Waverly Murder Suicide, 2012 4 Dead. 3 dead by asphyxiation, 1 dead by hanging Hunt family murders, 2014 5 Dead by firearm Sydney Siege, 2014 3 Dead, 4 wounded by firearm Cairns Child Killings, 2014 8 Dead, 1 wounded by stabbing Biddeston Murders, 2015 4 Dead by Firearm Fernando Manrique murders, 2016 4 Dead by gassing January Melbourne Car Attack, 2017 6 Dead, 30 injured by car attack Margaret River Murder Suicide, 2018 7 Dead by firearm Bedford Killings, 2018 5 Dead by stabbing and bludgeoning