To bill for an anaesthetist who assisted at a procedure use Item 25205 (for elective work) or 25200 (if patient is in imminent danger of death).
See https://www9.health.gov.au/mbs/fullDisplay.cfm?type=note&qt=NoteID&q=TN.10.9

Where applicable, you can also enter any any other modifiers in the range 25000-25014, and any associated therapeutic or diagnostic services in the range 22002 -22051.

For after hours assistance at anaesthesia in addition to the above items add 25030, CE will add a 50% loading. See MBS 25030