What you need to consider here is that proving beyond reasonable doubt that the interstate workshop in question is at fault is no easy task, at law, in court, it would be your word against theirs and a magistrate would most likely conclude that as the exact cause of the fault cannot be properly ascertained, the interstate workshop is not liable for any labour sums outstanding. No doubt, the interstate workshop will claim that your workshop was somehow negligent and ought to have properly installed the item in question. Point being, no point trying to get them to pay for labour, it is perhaps better to look at the nature of the item in question.
Assuming the item is not covered by an explicit warranty, it is perhaps worth claiming on statutory grounds (as per the Fair Trading Act or Trade Practices Act), that is you can try and enforce an implicit warranty, you could argue that the item is not of merchantable quality (i.e defective or that it is inherently faulty), or that it is not fit for the purpose that it is purported to be fit for, at best, the workshop would be liable for repairing the item, or replacing it. This can be done via the small claims tribunal (department of fair trading). Again, the outcome of such action would be subjective and dependent on many factors. That said, this is the better option, you have a chance of success, whereas, with claiming costs of labour you don't even have that IMO.