Defence lawyer Melinda Taylor is one of four members of the International Criminal Court’s staff detained in the Libyan town of Zintan since Thursday.
Locals are suspicious about documents she tried to pass to Saif al-Islam.
ICC president Sang-Hyun Song has urged Libya to release the four immediately.
On Sunday a spokesman for Australia’s foreign ministry confirmed the identity of the Australian lawyer detained as Ms Taylor.
He said consular officials were seeking access to her, as well as “clarification from the government in Tripoli on the circumstances of her detention”.
Libya’s representative at the ICC, Ahmed al-Jehani, said Ms Taylor was “under house arrest in Zintan, not in prison”.
The ICC delegation travelled to Libya on 6 June, to meet Saif Al-Islam Gaddafi in Zintan, “in part as a privileged visit by the Office of Public Counsel for the Defence, currently appointed to represent Mr Gaddafi in the case brought against him”, Mr Song said earlier.
“These four international civil servants have immunity when on an official ICC mission. I call on the Libyan authorities to immediately take all necessary measures to ensure their safety and security and to liberate them,” the ICC president added.
Saif al-Islam, who was captured last November by militiamen as he tried to flee the country, has been indicted by the ICC for crimes against humanity.
Libya’s interim government has so far refused to hand him over for trial in the Netherlands – the seat of the ICC. Libya has insisted he should be tried by a Libyan court.
Members of the brigade holding Saif al-Islam say the ICC team were carrying documents including a letter from a former confidante of his who is now in Egypt, the BBC’s Rana Jawad reports from Zintan, about 135km (85 miles) south-west of Tripoli.
The brigade commander said the ICC officials would remain in detention in Zintan pending an investigation.