Travel policy & per diem allowances

Hi all - we're looking at reviewing the Group travel policy, which among other things specifies a maximum 'per diem' allowance - what is reasonable to spend on meals (breakfast lunch and dinner) plus incidentals per day when travellign within Australia and SE Asia. Does anyone have some typical amounts (ours is currently AUD100 per day in Australia - this isn't supplier pricing so not commercially confidential). Also is per diem a good idea or does it just encourage people to spend up to the maximum - any alternatives to per diem?