Okay, that's good information. From what you've provided, I don't think that the Tribunal actually treated the respondent as an "expert witness". The procedural guidelines for expert testimony is "Procedural Direction 3 - Expert Evidence", which you can find at
Procedural Directions.
This direction only applies to the Consumer and Commercial Division in certain circumstances, or if the Tribunal itself makes such a direction. (See para 11.) Expert evidence also has to be provided impartially (paras 14-17).
It doesn't appear that the Tribunal treated the respondent as an expert witness, but it does appear that it allowed the respondent to give testimony of a technical nature - in other words, testimony based on the respondents knowledge as a VET, that should have been procured from an independent expert.
In most cases, NCAT makes it's own rules in regard to evidence as per s38 of the NCAT Act. Note paragraph 2 which reads:
"The Tribunal is not bound by the rules of evidence and may inquire into and inform itself on any matter in such manner as it thinks fit, subject to the rules of natural justice."
The question here is therefore whether or not the Tribunal accepted evidence from the respondent which should have been provided by an independent expert, and whether or not this was appropriate considering that the respondent obviously had a vested interest in the case. It's therefore a question of whether or not "natural justice" has been served under s38 of the NCAT Act.