Australian journalist Peter Greste remains in solitary confinement in a Cairo prison for at least another 15 days at the direction of a state prosecutor.

Egyptian authorities have extended the detention of Australian journalist Peter Greste on suspicion of breaching the country’s security.

The al-Jazeera reporter spoke to his Brisbane-based parents Lois and Juris early on Friday from Cairo’s maximum security Tora Prison, where he was told by prosecutors his custody would continue for at least another 15 days.

“It is incomprehensible to us that somebody can be in prison, solitary confinement … without any charges whatsoever, for this many days,” Lois Greste told AAP after the phone call with her son.

Greste, 48, and two of his al-Jazeera colleagues were arrested in a city hotel on December 29 on suspicion of broadcasting false news in the service of the blacklisted Muslim Brotherhood.

The Brisbane man, who works as East Africa correspondent for the global network, was due to be in Egypt for only a short time, relieving an absent colleague.

“He is very concerned about his two co-workers,” Ms Greste said.

“He has had no contact with them and believes they are being held in a different section of the prison.”

Al-Jazeera has arranged legal representation for the three men, but Greste is yet to appear in court and has only been brought before a state prosecutor.

“Under Egyptian law … prosecutors can make 15-day extensions three times and this is the third time,” Ms Greste said.

“After that it, if he hasn’t been charged, (the detention) can be extended by another 45 days for a period of up to two years.”

Despite receiving bad news she said her son remained in “reasonable” spirits and was overcoming a head cold.

“He’s been in tough places before,” she said.

“I feel confident he’s going to ride this out provided it doesn’t continue forever and a day.”

Greste’s parents deny reports from Australia’s Egyptian embassy that the men have been charged.

Egyptian ambassador to Australia Hassan El-Laithy told the ABC Greste’s arrest is not personal.

“But it is whether those working for a specific television station are abiding by the laws of that specific host country or not,” he said.