He was rearrested in 2007 after serving his sentence on an immigration warrant.
Earlier this week, a high court judge upheld an earlier extradition ruling by magistrates against Mann.
Samkange was shown documents on Friday, confirming that Mann had already been deported, on arriving at court to file papers relating to Mann's final appeal.
The deportation order had been already carried out at the behest of Bharat Patel, acting attorney general, who was a judge at the high court ruling.
"The attorney general is the one signed who signed this deportation order. He knew we were filing a notice of appeal but he failed to disclose that they had deported him," said Samkange.
A former member of Britain's Special Air Services (SAS) elite armed force, Mann was accused of touching down at Harare to collect weapons en route to Malabo as part of the alleged plan to oust President Teodoro Obiang Nguema Mbasogo, .
Mann says he and his co-accused were on their way to the Democratic Republic of Congo and that they needed the weapons for a security contract at a mine.
He was sentenced to seven years in jail, but the term was later reduced.
Eleven other men are serving sentences of between 13 and 34 years in Equatorial Guinea in connection with the alleged plot.
Mark Thatcher, the son of Margaret Thatcher, former British prime minister, was arrested in August 2004 on charges that he allegedly helped finance the coup attempt.
Thatcher pleaded guilty in South Africa to unwittingly helping bankroll the plot and was fined about 400,000 euros ($510,000).