The sole survivor of a tragic boating accident has reportedly committed suicide after being victimised by a hate campaign.

Matthew Dunn, 22, was found hanged under a bridge at Ashcroft in Sydney's south-west on the day of his daughter's 6th birthday party, according to News Limited.

Dunn was in a boat with his cousin and two friends when it capsized in the St George basin in September last year.

No one on board was wearing a life jacket.

Dunn was rescued by a local man who paddled out on a surfboard after hearing screams in the night.

Dunn's family told reporters he had been suffering from depression after receiving a bullet in the mail and being told "his time would come".

Dunn and his cousin Chris Lewis were more like cousins than brothers, his family told reporters.

"When we lost Chris we lost Matt as well, we lost his soul," cousin Hayley Lewis said.

"He was a shell, he just wasn't Matt anymore."

Another relative took to Facebook to condemn the 'hate campaign' against Dunn.

"To the people who drove my cousin to take his own life, you have murdered a poor young man who had had enough," Kate Lewis posted.

Police are understood to be investigating the factors leading to Dunn's depression and subsequent death.

An inquest into the boating accident will be held later this year.

If you are concerned about the mental health of yourself or a loved one, seek support and information by calling Lifeline 13 11 14.

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