The bizarre world of people who claim the Port Arthur massacre was a "conspiracy".

This year marks 20 years since the Port Arthur massacre, the largest mass shooting in Australian history and still one of the deadliest ever in the world by a single shooter.

Martin Bryant is currently serving sentences totaling more than 1,035 years for killing 25 people and wounding more than 23 others.

There were countless witnesses, multiple confessions and yet some people still insist there’s more to the story.

Some people actually believe Prime Minister John Howard somehow masterminded the whole thing to justify an anti-gun agenda.

What we know is that on the April 28, 1996, a 28-year-old lone gunman went one a murderous rampage through the historic Port Arthur prison colony, a popular Tasmanian tourist spot.

Survivors still live with the emotional and physical scars, families still grieve and, thankfully, Australians remain protected by a spate of strict gun control laws implemented soon after.

Sadly, as with any large-scale tragedy, Port Arthur is not immune to conspiracy theories.

Prime Minister John Howard in 1996.

A new film by writer/producer Paul Moder promises a "balanced and respectful" account of the tragedy — although it was made without the blessings of the victims families.

While Moder insists Bryant — the Port Arthur Massacre will not be exploitative, it will be "a hard hitting, no-holds-barred depiction of the massacre that will be very confronting and, by way of the subject matter, horrific."

"I will be including the anomalies, factual discrepancies and legal improprieties of the case in a balanced and mature manner and will include the 'grassy knoll' moments espoused by conspiracy theorists," he told the Sydney Morning Herald.

Paul Moder insists his film will give a "balanced" view of events. Source: Facebook

So what is a "grassy knoll" moment? It's a term most often associated with conspiracy theories surrounding the shooting of JFK, specifically that the shots which killed the US President came from multiple shooters in a number of location, including a nearby grassy knoll.

So what is a "grassy knoll" moment when it comes to Port Arthur?

Moder says there are a few:

"There are things like, because there was no open trial, there were never any fingerprints taken from the scene that proves that Martin was there, there was never any ballistic evidence matching the firearms to the crime scene or Bryant to the firearms," Moder tried to explain on the Project this week.

Aside from small discrepancies in accounts given by Bryant of the events and witness testimonies, (which can easily be explained by mental illness in his case and by trauma  in theirs), there are also other 'facts' frequently cited by conspiracists.

For example, in 1988, the NSW Premier Barry Unsworth said it would take a massacre in Tasmania before we could tighten gun laws. Or the fact that John Howard's gun legislation was drafted before the massacre occurred - both of which can be put down to common sense.

One website also claims senior staff at Port Arthur were conveniently away at a work seminar on the day of the shooting.

Conspiracy theories also allege that Martin Bryant couldn't have done it because he is left-handed.

But as Project host Waleed Aly pointed out this week: "If there was a defensible case, in this case, it would have run to trial."

We'll let him take it from here: