Moamar Gaddafi's son Saif al-Islam will be hanged if he is tried in Libya, his Australian lawyer has told International Criminal Court (ICC) in The Hague.

While the ICC wants Saif, the only son of the slain Libyan leader in custody, to be tried in The Hague, Libya's post-revolutionary authorities insist he should stand trial in his home country.

Libya's lawyers told a three-judge bench the country had enough evidence to charge Saif with crimes against humanity, committed when Gaddafi and his loyalists tried to put down Libya's bloody revolution last year.

But the lawyers admitted that although Tripoli was committed to a fair trial for Saif, it was a "complicated process and that Libya needed more time".

Australian lawyer Melinda Taylor, a member of the ICC's defence office, said Saif's right to a fair trial was being violated while he was being held in isolation in the north-western Libyan hill town of Zintan, where he has been in custody since his arrest on November 19.

Ms Taylor and three of her ICC colleagues were arrested on spying allegations earlier this year before an apology from the ICC saw her released.

"Although the Libyan government has danced around the issue, let's be very clear; if convicted (in Libya) Mr Gaddafi will be hanged," Ms Taylor told judges.

The ICC issued arrest warrants for Saif, 40, and the late Libyan leader's former spymaster Abdullah Senussi, 63, in June 2011 on two counts of crimes against humanity committed while trying to crush the uprising that ended over four decades of Moamar Gaddafi's iron-fisted rule.

But the ICC's jurisdiction is complementary to that of national courts, and it can only act when a member state is unwilling or unable to do so.

Ms Taylor cited a law passed by Libya's post-revolutionary National Transitional Council (NTC) which said no child of Gaddafi will ever benefit from leniency.

Putting Saif on trial will "not be motivated by a desire for justice but a desire for revenge," Ms Taylor said.

"Mr Gaddafi is not a guinea pig [for Libyan justice]. He is a person with rights," she said.

"He should not be languishing in prison while Libya tries to build a judicial system."

Libyan officials had asked in May for the court to quash a surrender request and throw out the case. The court's judges are to make a ruling at an unspecified later date.

Asked to clarify dates for Saif and Senussi's possible trials, Philippe Sands who represents Libyan authorities in the case, said "the best estimate for a start date is February 2013".

The UN estimates that up to 15,000 people were killed in the conflict, but Libya's NTC put the figure as high as 30,000.

AFP