Protesters claim defence stripped them

Defence has launched an investigation into allegations a group of arrested peace protesters were stripped and threatened with rape and drowning.

Army chief Lieutenant General David Morrison said four protesters were detained on October 2 by defence personnel and another four by Victorian Police, after breaking into the Swan Island defence base near Queenscliff, Victoria.

They've been charged with trespassing on Commonwealth land.

The protest at Swan Island is a regular event staged by the Swan Island peace group, objecting to the US alliance and Australian involvement in Iraq and Afghanistan.

Greens senator Scott Ludlam he had been told those arrested were "black-bagged", stripped naked, dragged along the ground, threatened with anal rape and with drowning.

Lt Gen Morrison said those allegations were taken seriously by defence and would be investigated to ensure actions undertaken by defence personnel were appropriate.