The robbers fled with around $1500 in cash, two cell phones, a laptop, an iPod and various items of jewellery.
The four were pursued by police, and Watene was caught by police dogs in a bush reserve, where the shotgun and various other items were also recovered.
He argued he was only the driver, and had unwillingly taken part to pay off a debt incurred by his daughter.
"He said his daughter had gotten into some trouble with the other offenders and they came to him and asked if he wanted to earn some money to pay them back, so he went along with them," the Court of Appeal ruling said.
"During his police interview he said, 'the guy who used to go [out] with my daughter asked if I wanted to make some money. I didn't know they had a gun, I was only the driver'."
However, "several incriminating texts" were found on his mobile phone.
He was identified as the shortest of the four robbers, with the three victims describing him as doing "all of the talking and the giving of orders", as well as making threats.
Watene was jailed last year for his role in the aggravated robbery.
He appealed against both his conviction and his sentence, claiming he did not receive a fair trial and that the judge used a starting point for sentencing that was too high.
However, the Court of Appeal ruled that none of his arguments held up, and dismissed both appeals.