Australian journalist Peter Greste’s appeal of his conviction and seven-year jail sentence is a necessary move towards any early release, an expert says.

If Australian journalist Peter Greste’s appeal against his conviction and sentence is unsuccessful, it appears the move will at least open the door to a possible early release or pardon.

The al-Jazeera reporter and his family launched the legal challenge on Friday, just more than a month after an Egyptian court sentenced him to seven years’ jail for falsifying news and aiding the blacklisted Muslim Brotherhood.

An international law expert says while the move is Greste’s only real legal option, it’s also a necessary step towards any diplomatic requests for clemency.

Egyptian President Abdel Fattah al-Sisi, who won a presidential election in May, cannot intervene to pardon Greste until the appeals process is completed.

“It’s important that Peter Greste demonstrate that he has exhausted all of his opportunities to appeal his sentence before the Egyptian courts,” Professor Don Rothwell of the Australian National University told AAP.

“If he’s successful on appeal, well, that’s a good outcome. If he’s not successful on appeal, well, that doesn’t close off options for the Australian government and gives those options greater strength.”

Greste’s youngest brother, Mike, said the family had sought extensive legal advice before deciding to appeal the June 23 verdict and sentence, which had shaken the family “to the core”.

He said it was unclear how long the appeal process would take.

“We have to have faith in the Egyptian system,” Mr Greste told reporters in Brisbane on Friday.

“The research we’ve conducted, it’s clearly the most apparent course of action to try and secure his freedom.”

The journalist’s Canadian-Egyptian colleague Mohamed Fahmy was jailed for seven years and Egyptian al-Jazeera reporter Baher Mohamed was jailed for 10 in the same Cairo court.

From the city’s Tora prison Peter Greste and Baher Mohamed wrote how the guilty verdicts had hit them like a body blow, rendering them oblivious to the uproar that erupted in the packed court.

“There was a nausea, the gasping for air, the shaky knees, the oppressive weight of the concrete and iron cell weighing down on our shoulders with a heaviness we had never felt before. Seven to 10 years for crimes we did not commit,” they said in a joint statement on Friday.

The prisoners urged supporters around the world to maintain pressure on Egypt to free them.

“As long as we remain behind bars, all of Egypt’s press works with the threat of imprisonment hanging over it, and the nation’s fledgling democracy wears a muzzle,” they wrote.

The court released a statement on Tuesday saying “the devil guided” the group to spread false news defaming the country.

Egyptian authorities have been incensed by the Qatari network’s coverage of their deadly crackdown on supporters of Islamist president Mohamed Morsi, who was ousted by the army in July 2013.