Short answer:
The assisting anaesthetist bills
1. 25205
2. Modifier units
3. Time unit
No preassment items.
Only one anaesthetist can bill for invasive monitoring or insertion items.
"MBS Item 25205 applies to assistance from a second anaesthetist for elective anaesthesia where one or more of the following conditions are present: complex airway problems, neonate or complex paediatric case, anticipated massive blood loss, a critically ill patient or where total anaesthesia time exceeds 6 hours (for more information see Note T10.9 on pp 247 and 248 of the Nov 2007 MBS book ). "
http://www9.health.gov.au/mbs/fullDisplay.cfm?type=note&qt=NoteID&q=T10.9
http://www.asa.org.au/UploadedDocuments/RVG%20FAQs.pdf