You do not need to appoint a Surveyor. The Owners can agree matters in writing between themselves if they are able to do so and, presumably, if they are satisfied that their interests are being properly protected.
Amongst many other duties, appointed Surveyors are authorised by Section 10 of the Act to determine in an award:
The Surveyors will usually, where possible, prepare a schedule of condition of the adjoining owners property before any work starts. In the event of damage, or alleged damage, caused by the works, the schedule will be of assistance in determining whether the notifiable works did actually cause damage.
The Surveyors will prepare, agree and serve an award that covers important aspects of the works such as working hours, access and safeguards to reduce the risk of damage occurring and they will provide a method of resolving disputes between the owners arising from the works.
The Surveyors more often than not, will retain the right within an award to make and serve further awards (eg: should damage occur, if the scope of notifiable works is required to be materially altered)
