Three of five men convicted for their part in New Zealand's biggest aggravated robbery have had their sentences reduced one year by the Court of Appeal.

But the mastermind behind the heist failed in his bid for a lesser sentence.

The gang took about $940,000 during the robbery of two Chubb security guards as they went to load an ATM in Willis St, Wellington, on December 22.

The ringleader, Peter Richard Tyson, aged 26, John Rauhina Moeke, 25, and a sixth man who has not appealed against his 9 1/2-year sentence, Craig Anthony Ferris, 24, of Christchurch, were armed with an air rifle, a crowbar and a club when they jumped the guards in a yard behind Willis St.

The three other men, Jonathan Robert McDonald, Quintin William O'Brien and Wayne Turner, played lesser parts, acting as lookouts, conducting surveillance before the robbery and escorting the van to its hideout, at the historic Kaiwarra Magazine building in Ngaio Gorge, after the heist.

In yesterday's decision, issued by Justice John Doogue, the court quashed the 7 1/2-year sentences imposed on McDonald, 27, O'Brien, 23, and Turner, 29, and replaced them with sentences of 6 1/2 years.

The court found their original sentences did not reflect the true difference between their roles and those of the other men involved.

Tyson, an unemployed security guard from Upper Hutt, received a total sentence of 11 years for the robbery, the arson of the Kaiwarra Magazine, which was destroyed when the security van was set alight, and an unrelated theft.

Moeke received an eight-year sentence.

Tyson's lawyer, Bruce Davidson, told the Court of Appeal that Tyson had received no credit in his sentence for his efforts in helping to recover $676,000 of the stolen money.

But about $258,000 remained outstanding.

An unknown amount was burnt during the magazine fire.

The court found that despite Tyson's cooperation with police and his early guilty plea, the 11-year sentence was not manifestly excessive.

"Given, however, the seriousness of the three distinct criminal events for which Tyson had to be sentenced, no lesser penalty could have been imposed."

Moeke's eight-year sentence was also not manifestly excessive, the court said, and reflected the role he played in the robbery.