A three-person panel of assessors -- similar to jurors -- unanimously cleared former Land Force commander Pita Driti of allegations he led a 2010 plot to overthrow the military regime and assassinate its attorney-general, the Fijivillage news website reported.
But it said High Court judge Paul Madigan overturned the assessors' decision and declared Driti guilty of inciting mutiny, saying the ex-officer had given "evasive" and "contradictory" evidence in his defence.
"I find beyond reasonable doubt that at the relevant times he was making an attempt to persuade the (Tagicakibau) from his loyalties to both service and state," it quoted the judge as saying.
Driti is set to be sentenced on December 10.
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In the Fiji legal system, three ordinary citizens act as assessors to weigh up the evidence against the accused in serious criminal matters. However, unlike Western-style jury systems, the presiding judge has the right to overturn their verdict.
Tagicakibau said Driti complained that Attorney-General Aiyaz Sayed-Khaiyum, widely seen as Bainimarama's number two, had too much influence on the military leader and should be killed.
He allegedly wanted to depose Bainimarama in October 2010 while he was in Sudan visiting Fijian troops serving as UN peacekeepers.
Tagicakibau said he and two other officers blew the whistle on the plot just before Bainimarama's scheduled departure for Sudan.
Fiji has experienced four coups since the mid-1980s, largely stemming from tensions between indigenous Fijians and ethnic Indians brought over by Britain in the colonial era to work on sugar plantations.