Outgoing Fiji military chief Mosese Tikoitoga Monday said the nation's armed forces were stable despite his sudden career switch.

The Fiji government announced Tikoitoga's resignation at the weekend, saying the military boss wanted to start a new life in the foreign service.

Movements within the Fiji military are closely monitored in the South Pacific following four coups in the island nation in the past 28 years.

The opposition Fiji Labour Party questioned the sudden resignation following reports of dissatisfaction in the military.

Labour Party president Lavinia Padarath said the opposition was concerned about comments made by acting military commander Viliame Naupoto that Tikoitoga was "not untouchable".

Tikoitoga, who was appointed head of the military only 18 months ago by 2006 coup leader Voreqe Bainimarama, rejected talk of tension within the armed forces and said he had been thinking of a career switch for some time.

"It's most unfortunate that these rumours are spread. But they are all false. As you know the honourable prime minister (Bainimarama) is very much in charge," Tikoitoga said on Monday.

"In the last few months I have been contemplating going on a diplomatic post. There's always been a personal mission of mine that I can be considered for a diplomatic post."

Naupoto, named as acting commander, has been promoted from captain to commodore "with immediate effect", the government statement said.