Anyone! Well, anyone who owns or co-owns the residential land/premises where building work (over a certain value which can differ by state) is going to be done.
Plus, anyone who is prepared to:
undertake an Owner Builder course;
take on the duties and responsibilities of a building project;
take on the risks (know the risks);
understand the building process;
go through what can sometimes be a painful process of getting the required permits and certifications;
take on the mental, physical and emotional challenge