Building expert witness is someone with expert knowledge that will not only give evidence in the VCAT or in the court but is also privilleged to give an opinion. If you think of your case as a chain then your expert witness is the bigest link even though your chain is only as strong as its's weakest link.
The problem with building disputes is that they are complicated and require of high level of technical and regulatory knowledge. Lawyers on the other hand are experts in legal proceedings but are not building experts and will heavily rely on your building expert witness. This is why your dispute will most likely be decied on whether your expert's opinion or your oponent's expert prevails.
A point often not understood is that interpretation of building control in Victoria is not vested in the lawyers but in the Building Surveyors. They are the ones that interpret the Building Act, Building Regulations, BCA and the Australian Standards.
So your choice should be building expert that also has qualifications in building Surveyoing. This is what appears in my expert reports:
  1. A statement identifying the experts area of expertise:
Particular knowledge of all types of low-rise construction, knowledge of all trades and including professions of: Quantity Surveying, Estimating, Project Management and Supervision. By virtue of the best qualifications and extensive experience, expertise is claimed in the following areas:
  1. Building Technology
  2. Building Practice
  3. Project Management
  4. Contract Management
  5. Building cost and estimating
  6. New home inspection
  7. Property pre purchase inspections
  8. Building control and regulations
  9. Dispute management
  10. Expert witness
  11. Forensic investigations of building defects
  12. Forensic investigation of project documentation (paper trail)
  13. Thermal imaging for diagnosis of building defects
  14. Building reports, special technical/investigative reports on any subject
It is also important to back up claims of expertise with academic qualifications and long history of work experience as well as the history of appearance as expert witness:

Degree in Building, majoring in Quantity Surveying from the
University of Melbourne: graduating in 1976. The above qualifications have been updated to include a pass in Construction Regulations A and B at the same university in 2005-6.
Diploma in Building Surveying: Gordon TAFE 2006.

Over 40 years of practical experience as a domestic and commercial builder

For example, in a recent dispute my client was refused insurance claim on the basis that carport roof was blown over because of rot in timber members and that wasnt covered in the policy. I have been able to establish that there was no rot and the insurer's expert was not in fact an expert and was not qualified to make claim about structural rot. I am qualified and accredited timber pest inspector and I was able to trump him. Insurer buckled and allowed the claim.

Your choice of expert witness is the most important part of your dispute, choose wisely.
Trying to save a few hundred dollars with a cheaper building consultant will be like trying to plug a leak in your boat with a chewing gum, your case could sink.