By Agence France-Presse
James Ricketson has been held in jail since his arrest in June last year after he flew a drone over a rally held by the opposition Cambodia National Rescue Party (CNRP), which was dissolved months later.
The CNRP's dissolution paved the way for strongman premier Hun Sen to win a clean sweep of all parliamentary seats in July's national election, which Western democracies have said was flawed in the absence of a viable opposition.
After a six-day trial, Judge Seng Leang found the 69-year-old Ricketson guilty on two charges of espionage.
"We have decided to convict (him) to six years in prison for espionage and collecting harmful information that could affect national defence," he said.
The prosecution had accused Ricketson of working as a filmmaker in Cambodia for years as a front for spying.
"Unbelievable -- which country am I spying for?" Ricketson asked out loud in court.
His lawyer Kong Sam Onn told reporters waiting outside the court that he plans to request a royal pardon from the Cambodian king.
Earlier this week 14 opposition lawmakers and activists jailed before the election were released after sending apology letters to Hun Sen, which the premier said he sent on to the monarch.
Calling the result "devastating", Ricketson's son Jesse said he could not comment on whether an apology letter to Hun Sen was forthcoming to secure his father's release.
Human Rights Watch's Phil Robertson decried the court's findings on Friday, saying that the trial "exposed everything that's wrong with the Cambodian judicial system".
Robertson said the Australian was used as a "scapegoat" by the government to crack down on political opposition.
He also criticised what he said was inaction by the Australian government in "failing to publicly and consistently challenge this ludicrous charade and demand Ricketson's immediate and unconditional release."
In the months leading up to the election, the Hun Sen-backed government cracked down on opposition lawmakers, journalists and activists.
Ricketson has faced legal problems in the past. He was handed a two-year suspended prison sentence in 2014 for allegedly threatening to broadcast allegations that a church working in Cambodia had sold children.
Two years later, he was fined after a court found him guilty of defaming an anti-paedophile NGO by accusing the group of manipulating witnesses.