In an emotional letter which he sent from Tora jail, on the outskirts of Cairo, where he is being held, Mr Greste said he had decided to speak out in defence of the fundamental right of freedom of the press, having realised that his arrest was not due to a mistake but a deliberate campaign by the government.

Greste and two producers, Mohamed Fahmy and Baher Mohamed, are accused of collaborating with a terrorist organisation [the Muslim Brotherhood], of hosting Muslim Brotherhood meetings in their hotel rooms, of using unlicensed equipments to deliberately broadcast false information to further their aims of defaming and discrediting the Egyptian state.

Mr Greste said the Egyptian Government had presented no evidence to support the allegations and he had not been formally charged with any crime.

But the group's initial 15-day detention was extended by another 15 days two days ago and further extensions are possible.

‘‘Our arrest and continued detention sends a clear and unequivocal message to all journalists covering Egypt, both foreign and local,’’ Mr Greste wrote.

‘‘The state will not tolerate hearing from the Muslim Brotherhood or any other critical voices. The prisons are overflowing with anyone who opposes or challenges the government. Secular activists are sentenced to three years with hard labour for violating protest laws after declining an invitation to openly support the government; campaigners putting up "No" banners ahead of the constitutional referendum are summarily detained.

‘‘I have no particular fight with the Egyptian government, just as I have no interest in supporting the Muslim Brotherhood or any other group here. But as a journalist I am committed to defending a fundamental freedom of the press that no one in my profession can credibly work without,’’ he wrote.

Mr Greste said the dilemma for a journalist was how to obtain political balance in reporting on Egypt.

‘‘The Muslim Brotherhood has lost much of the support and credibility once had when its political leader Mohamed Morsi became Egypt’s first democratically elected president just over a year and a half ago. And many here hold it responsible for a growing wave of islamist violence, but it remains the single largest and best organised social and political force in Egypt. What then for a journalist striving for "balance, fairness and accuracy?" How do you accurately and fairly report on Egypt’s ongoing political struggle without talking to everyone involved?’’ he wrote.

Peter’s father, Juris Greste, said the family had spoken to him three days ago, just after midnight, on a consular officer’s phone when the news came through that his detention was being extended for a further 15 days.

‘‘The impression he gave us was that he was OK. He’s bearing up as well as can be expected,’’ Mr Greste senior said.

‘‘This is definitely the worst thing that has happened to us in our lives,’’ he said.

Federal secretary of the the Media Entertainment and Arts Alliance, the union representing Australian journalists, Chris Warren said Mr Greste and his colleagues from Al-Jazeera English language service, were being made an example of because of perceptions within the Egyptian government that the Arabic Al-Jazeera was supportive of the Muslim Brotherhood.

‘‘Obviously, what we are seeing is a serious crackdown on freedom of speech that its affecting both Egyptian and foreign journalists reporting from Egypt,’’ he said.

Mr Warren said the union was in regular contact with Mr Greste’s family and last week had presented a petition with 1000 signatures of journalists to Egypt’s ambassador in Australia.

Mr Greste, who had been in Egypt for only two weeks, said he had worried about how to fairly report the conflict with his producer but decided that ‘‘the choice was obvious – as obvious as the price we are now paying for making it.’’

Mr Greste said he was being kept in a ‘‘cold’’ prison cell 24 hours a day and was allowed out only for visits to the prosecutor for questioning. He had just been allowed out for his first exercise session in the grass yard behind the cell block.

But Mr Greste said he was more worried about his colleagues.

‘‘Fahmy and Baher have been accused of being Muslim Brotherhood members, so they are being held in the far more draconian "Scorpion prison" built for convicted terrorists,’’ he wrote.

‘‘Fahmy has been denied the hospital treatment he badly needs for a shoulder injury he sustained shortly before our arrest. Both men spend 24 hours a day in their mosquito-infested cells, sleeping on the floor with no books or writing materials to break the soul- destroying tedium.’’

Mr Greste’s father said he understood that one of the producers had now begun a hunger strike.

He said Australia’s consular officers and staff of Al Jazeera were providing food for his son while in prison.