Effectively, they are secure at all times as they are at an airport. Anyone accessing or requiring access to the aircraft has been security checked and background checked to even be allowed anywhere near it.

But, in terms of somewhere like SYD, the aircraft will be remotely parked, and the steps removed. There is also a sticky security seal that can be places across the door, and if the door is opened, it pulls part of it away, telling staff responsible for it, that is has been opened. A bit like those stickers you get on a new mobile phone box where you open it and the words "VOID" then appear on the sticker etc...