Joe Magarey – Activists of the "Freedom Flotilla" left Thursday Island yesterday bound for disputed waters off West Papua and a probable ocean confrontation with Indonesian forces, defying a "final warning" that they could be jailed in Indonesia for 20 years.

The six sailors who will take the final, perilous leg of the journey expect to reach Indonesian waters within two to five days, as reports emerge of increased troop movements and vessel interceptions at their destination port.

The venture looms as an early test for foreign minister designate Julie Bishop, who last night called for a "proportionate response" from Indonesia after accusations she had potentially put "Australian lives in danger" with earlier comments.

An Australian government official at a Thursday Island Customs office delivered a high-level government warning that the activists could face 20-year jail terms in Indonesia.

The official said the activists could be jailed for five years if charged for not having correct visas or permits or for the longer terms if prosecuted for "crimes against the security of the (Indonesian) state".

Greens senator Richard Di Natale has released a scathing letter to Ms Bishop in which he claimed comments by her that Indonesia was "entitled to use whatever means it wishes" in responding to the activists had potentially put their lives in danger.

Last night, Ms Bishop said while the protesters planned to "deliberately breach Indonesia's territorial sovereignty", she "would urge Indonesian authorities to respond proportionately".