More than a year after Tej Chitnis disappeared without a trace, his family say they continue to hold hope that he's still out there.

Chitnis, who went missing in April 2016, is one of the faces of National Missing Persons Week, which launched on Sunday.

Near Melbourne's Queen Victoria Market, there's a mural of Chitnis, which was curated by charity group the Missing Persons Advocacy Network and serves as a reminder about the more than 2,000 people who are categorised as long-term missing.

Each year, a total of about 38,000 Australians are reported missing, two thirds are under the age of 18. Most are found between 48 hours and a week after they're reported missing.

AAP

It was April 26, 2016 when Tej Chitnis, then 21 years old, didn't show for his father's 60th birthday.

He told his parents, Jayant and Reva, not to leave for the party without him.

"The fact that he didn't turn up really raised the hackles, it was unheard of," Mr Chitnis says.

"It was a big celebratory dinner. My older son was going to go away to Africa to do some volunteer work for three months. So it was kind of a combined celebration, a going away get together for him, and my 60th birthday."

And so begun the family's desperate search for clues.

On the day he went missing, his phone pinged near Healesville, east of Melbourne, where his car, a silver Volkswagen Golf registration TTF517, was captured by CCTV footage.

Despite a Facebook campaign to find him, there's been no trace since.

“It's a nightmare," Mrs Chitnis says.

"It's a constant nightmare. It's the not knowing that's the nightmare."

Mr Chitnis describes the disappearance as a "big hole" that "keeps getting bigger", and you "try to numb it, try to get to work, but it's not easy".

Shortly after his disappearance, his parents learned that Tej had not attended his health sciences course at Deakin University for more than a year.

Mrs Chitnis says that information provided the family with with 'false hope'.

"Probably came as a relief to us, even though we're 'Indian parents', it came as a relief to us, to think, OK that's the reason he might have run away, kind of thing, when we found out.”

Chitnis missed his father's 60th and 61st birthdays, but his family say they still hold hope he will return home for the others.

"We love him, and it's OK to be away, just let us know that you're OK," Mrs Chitnis says.

Mr Chitnis adds: "If he's not ready to come back, it's OK. He can just tell us, I'm alright, I'll get back to you, whenever, it's fine with us. We want him to be OK wherever he is."

SBS

Australian Federal Police National Coordinator of Missing Persons Marina Simoncini says authorities are seeking help from the public to locate missing persons.

"We have seen cases of long-term missing persons in each state and territory, (in) Victoria and New South Wales of late, where people that have been missing for more than 30 years have been found," she told SBS World News.

Ms Simoncini says there are several reasons people may go missing.

"It could be simple miscommunication or misadventure, where police would launch a search-and-rescue operation," she says.

"It could be that they've legitimately escaped from violent situations, it could be drug or alcohol abuse, and in the most extreme of circumstances, they may have become a victim of crime."

For more information about the "Unmissables" campaign, visit the Missing Persons Advocacy Network website www.mpan.com.au