‘His lunch was in the fridge and his milk was dated May, that’s how we established the date he went missing.’

Irishman turned new Australian citizen Gerard Mooney was still settling into his Sydney apartment when he vanished without a trace.

Three decades later his family are still seeking answers about what happened to him.

It hasn’t helped that Mooney’s disappearance went unnoticed for several months.

He had only recently moved to Marrickville in Sydney, after nine years living in Melbourne, when he was last seen on May 14, 1996.

But the very short time the then 36-year-old had been in his new home meant there were very few people he knew locally who would miss him.

“When he didn’t show up for work, the boss just assumed he’d left and he never contacted anyone, neither police nor family,” Mooney’s sister, Jean MacDonnell, told 7NEWS.com.au.

“He had lived in Melbourne and had friends there … but none close enough (in Sydney) to report him missing.”

Mooney had first immigrated from his home in Dublin to Melbourne in 1987 and fell in love with Australia, later becoming a citizen.

MacDonnell said he would write home periodically and had returned for their mother’s funeral in February 1996 — the last time they would ever see him.

Months went by without any alarms being raised, until Mooney’s brother got married in July of that year.

“We hadn’t heard from Gerard since April, we called his number and the line had been disconnected,” MacDonnell said.

“We contacted the police and they gained entry to his apartment. His lunch was in the fridge and his milk was dated May, that’s how we established the date he went missing.”

Mooney’s passport was still in his unit — which MacDonnell said Mooney had only just moved into — and his bank accounts had been untouched.

The sole clue was his cherished racing bicycle — custom-made for his 190cm-tall frame — which was found abandoned near his home four days later.

Mooney was now officially reported as missing and his older sister rushed to Sydney to help the widespread search efforts.

But the case had already run cold and Mooney was declared dead by a coronial inquiry in 1999.

Despite decades of calls for help solving Mooney’s cold case, the lack of significant evidence has meant no progress has been made to provide his family any answers.

On the 30th anniversary of his disappearance his family still have “zero idea” what happened to him.

His father died in 2002, and MacDonnell has had two children who never got to meet their uncle.

MacDonnell is now appealing to “see if anyone remembers anything and can give us some closure”.

NSW Police said the case “remains open and is monitored for any new information that may become available to progress the matter”.

Anyone with information has been urged to contact Crime Stoppers.