An interstate convicted child sex offender, who was able to run a milk bar near a high school in Melbourne's south east and gain access to another young victim, has been jailed.

The man's previous crimes predate the national child sex offender register.

His sentencing this week has come as national policing authorities recently struck down a controversial proposal to make the register available to the public.

Brett Barrett, 53, re-established a long-dormant milk bar, metres from a high school, in late 2013, but the business, which also operated as a cafe and advertised "after-school specials," was open for less than six months before his arrest in February.

Weeks later, anonymous flyers began to appear around the neighbourhood and were plastered to the shop's windows, claiming Barrett was a convicted child sex offender. Police have not charged anybody in relation to the flyers and their origins remain a mystery.

On Thursday, Barrett was sentenced to seven months jail for three counts of committing an indecent act against a teenage victim, including groping the boy, showing him pornography and sending him a picture of his genitals. Barrett's lawyer indicated he would appeal the sentence.

The father of the victim told Fairfax Media Barrett's actions had seriously impacted on his son's sense of well-being. "He locks the house up like Fort Knox," the father said. "He won't catch the bus any more."

At Barrett's sentencing, the Melbourne Magistrates' Court heard that the boy had felt "powerless" during the offending.

Fairfax Media understands detectives investigating the case were not aware Barrett, who in the late 1990s was sentenced to two years' jail in Queensland for sexual offences against a teenage boy,  was running a milk bar.

The National Child Offender Register was adopted in Victoria in 2004 and anybody convicted of a sexual offence against a child since has faced ongoing reporting obligations and police monitoring, but the laws did not apply to many people whose crimes predate them.and do not appear on any register.

"Since the legislation kicked in, he would have been monitored and we would have known about it," said a detective not linked to the case. "These people have to tell us when they go interstate and if they're going for more than 14 days we have to notify the neighbouring state they've gone to. Even if the person doesn't tell us themselves, we'd know about it because of our regular checks."

Earlier this month, Australian police ministers and attorneys-general at a conference in Geelong, discussed the merits of a public sex offenders register, in which information on serious offenders would be made accessible to the community.

The proposal was struck down by an advisory group made up of officials from all Commonwealth, state and territory justice departments, and police commissioners' representatives, after the bulk of evidence presented showed public registers were ineffective and concerns were raised about the potential to incite vigilantism.

Among that evidence was a study of 136 serious sex offences in Massachusetts in the US, in which only six cases were found to have had any potential for a community notification to have an impact. The bulk of offences (100) had been committed by people who had no criminal history.

Michael Bennett, a spokesman for Victorian Police Minister Kim Wells, said the Napthine government's position on public registers had not changed following the conference.

"As a government we continue to strengthen current sex offender management laws in Victoria," Mr Bennett said. "Minister Wells has also made a commitment to monitor evidence on the efficacy of schemes involving public disclosure of sex offenders with a view to discussing that evidence at a future date."

The Northern Territory went out on its own after the conference and announced plans to introduce a territory-wide public register, which would publish the photos, names and locations of people convicted of serious sex crimes.

The bill would be called "Daniel's Law" named after Queensland boy Daniel Morcombe who was murdered by a repeat child sex offender, Brett Peter Cowan, in 2003. Ten years before taking Daniel's life, Cowan had attacked a six-year-old boy in Darwin.

Magistrate Amanda Chambers sentenced Barrett to an immediate prison term and imposed an 18-month corrections order that requires him to undergo treatment. She also said he would be added to the sex offenders register and face lifelong reporting obligations.