Emails sent to self-exiled former opposition leader Sam Rainsy have taken centre stage in the investigation against jailed Australian filmmaker James Ricketson, who was brought in for questioning this morning over correspondence that an investigating judge presented as evidence of “espionage”.

Ricketson, 68, was arrested in June last year after he flew a drone without a permit over an opposition rally. He was charged under espionage laws with gathering information for a foreign power that could damage national security, a charge he has denied.

Ricketson was questioned for two hours by the judge in a closed Phnom Penh Municipal Court session, and is expected to be questioned again next week, according to his lawyer, Peung Yok Hiep.

Yok Hiep said the judge focused on the content of three emails obtained from Ricketson’s seized computer. The emails were the first pieces of alleged evidence Ricketson has seen during his eight months of pre-trial detention.

— In an email to Rainsy from 2013, Ricketson allegedly asks for confirmation as to whether Rainsy was subject to a new arrest warrant. At the time, Rainsy was head of the Cambodia National Rescue Party, which was forcibly dissolved in November following the September arrest of his successor, Kem Sokha, on suspicion of "treason".

— In another email, Ricketson allegedly responds to an email from a foreigner inquiring about the number of bodyguards assigned to Prime Minister Hun Sen, and how many water cannon trucks – used to break up protests – the government has available. Yok Hiep did not disclose who sent the email, but said Ricketson responded by saying the number of bodyguards was about 10,000.

— In a third email, an Indian woman asked Ricketson to help a Cambodian film team with a project for an NGO, which Yok Hiep couldn't immediately identify.

Ricketson confirmed he had sent the email to Rainsy, as "it is normal, because he is a journalist, to meet politicians”, Yok Hiep said. “He just wanted to confirm with Sam Rainsy, but he was charged with collecting information.”

Rainsy defended Ricketson today, saying he was simply doing “his work as a journalist” in corresponding with him.

“Emails I received from him were of the kind I received from many other foreign journalists,” Rainsy said in an email. “Now James is only a scapegoat and a hostage held by the government to frighten other foreign journalists and deter them from writing critical reports about the Hun Sen regime.”

Ricketson is being held in the capital’s notoriously overcrowded Prey Sar prison, where he was recently moved to a cell containing 140 inmates.

Ricketson’s son, Jesse, was present at the hearing. His family has expressed concerns about Ricketson’s ongoing detention, due to his age and his health.

“He absolutely denies the charge against him. He’s baffled as to why this is all happening in the first place,” Jesse Ricketson said. “He acknowledges that he flew a drone and he acknowledges that he didn’t have a permit for flying the drone, but beyond that … [it] is a mystery to him and us.”

“They’re trying to see if there is any information in his emails that implicates him in spying for anyone, as it stands there isn’t anything in those emails. Hopefully through that questioning they’ll realise that they haven’t actually got any evidence.”

Additional reporting by Erin Handley