The accused gunman warned the man to "choose your words carefully" and "think of who you insult".
"I hope you one day see the light and if you are a Marxist I hope you one day meet the rope."
The recipient of the threat reported the incident to police in September 2016.
He provided a copy of the conversation and told officers he believed the accused gunman was dangerous, ABC reported.
The man said police did not take a formal statement – and instead told him to block the threat maker on social media.
After the Christchurch mosque attack that claims 50 lives the man was shocked to discover the identity of the alleged culprit.
"I feel a sense of guilt and have been beating myself up," he told the ABC.
"But at the time I thought I was the only one in danger."
Victoria Police said they had been "unable to locate a complaint made by the victim in 2016".
"Whilst we will not comment on this particular case due to the live court proceedings in New Zealand, we would reassure Victorians that we have strong arrangements in place for monitoring and tracking people who pose a threat to the community."