Changes are being considered for Australia's terrorism threat level system, as the nation's spy chief warns it wasn't designed for the "degrading security environment" facing society.Following his annual threat assessment on Wednesday evening, ASIO director-general Mike Burgess said the system in place should be reviewed, and that he is in talks with Home Affairs Department Secretary Stephanie Foster on how it could be overhauled.The nation's terrorism threat level is probable, meaning there is a greater than 50 per cent chance of an onshore attack, or attack planning in the next 12 months.But Mr Burgess said the level "does not tell...