Mellow's lawyer, Rob Vigor-Brown, said his client had pleaded guilty without a lawyer at his first appearance and had been "more than co-operative" with police.
He said Mellow, who had 40 previous convictions, was a meth addict.
"Obviously that was one of the drivers of his offending," Mr Vigor-Brown said.
He said Mellow wanted to address his addiction, but couldn't without counselling.
Judge Tompkins said Mellow had targeted tourists in a vulnerable situation, but noted it was opportunistic rather than premeditated offending.
He said he hoped the prison term would allow Mellow to get treatment for his drug addiction, which was a "driving force" behind the robbery.
Mellow was also sentenced to concurrent terms of imprisonment for two charges of unlawfully taking a motor vehicle, three of theft of petrol, theft of a jacket, two breaches of community work and release conditions, an assault and a breach of bail.
Mellow's alleged co-offenders Michael William Absolum, 33, and Blake John Lee, 19, have both pleaded not guilty to aggravated robbery and will next appear in court on September 11.