Both Roberts and Narayan had been charged in January last year under the Crimes Act and the Secret Commissions Act.
Roberts was the former asset manager for Te Roopu Taurima O Manukau Trust (Te Roopu) and trustee and employee of Te Kawerau Iwi Tribal Authority (Te Kawerau).
Te Roopu is a public healthcare provider for people with intellectual disabilities and Te Kawerau was set up to settle Treaty claims.
While acting as a trustee at Te Kawerau in 2009, Roberts received a secret payment of $45,000 in return for withdrawing public submissions he had lodged on behalf of Te Kawerau in opposition to a proposed change to a district plan.
The company that made the payment was unaware that Roberts was acting without the knowledge and consent of his employer, the Serious Fraud Office (SFO) said in a statement.
In 2012, while employed by Te Roopu, Roberts also received secret commission payments in return for contracting work to certain suppliers to Te Roopu, including businesses owned by Narayan.
Narayan, the owner of Pit Stop Otahuhu, was approached in January 2013 by Roberts who was responsible for the trust's vehicles, asking to have the car fleet repaired at the South Auckland garage.
However, between January 22, 2013, and April 24, 2014, Narayan began invoicing the trust for false jobs and withdrawing a cash kickback to give to Roberts.
At Narayan's sentencing, his lawyer, Ron Mansfield, said his client was "courted and seduced" by Roberts who "was clearly the instigator".
"Roberts, in my submissions, is bad news and has connections which caused [Narayan] to have concern on each occasion," Mansfield said.
However, Justice Woolford said Narayan's offending was "deliberate and protracted" and described him as a "willing participant" in the ruse.
Narayan was sentenced to six months' home detention for two counts of corruptly giving an agent reward or inducements and one charge of obtaining by deception.
SFO director Julie Read, who brought forward the prosecution, said the case was of significant public concern.
"Deliberate acts of fraud against a publicly funded healthcare provider and a charitable trust are completely unacceptable," she said.
Roberts will be sentenced next month.