Courts operate by specific rules in regard to the determination of whether evidence presented to them is admissible. These rules exist to help ensure fair and reliable outcomes in trials. Without rules of evidence, trial decisions would be arbitrary and unpredictable and the rule of law would be difficult to find. Lawyers, and investigators working for them, would have strong incentives to act unfairly.
When any item of evidence is adduced at trial, the other party can raise an objection to its adduction, arguing that it is inadmissible. Such an argument will stand if there is a problem with the source, the manner in which it was obtained, the subject matter, or the manner of its presentation to the court. Whether there is such a problem hinges upon specific rules, many of which were developed over centuries. The evidence objected to is ruled by the judge to be admissible or inadmissible. The party attempting to adduce the evidence can counter with an argument of their own and have the evidence admitted. Sometimes the question of admissibility is complex the arguments drawn out: so as to deal properly with a subtle or involved dispute a judge may convene a voir dire – that is, a trial within the trial, focusing solely on the question of the admissibility of evidence.
In recent times, in most states and territories, the common law rules of evidence and accompanying statutory laws have been replaced by the Uniform Evidence Law (UEL). Starting with a Commonwealth model Act, UEL is now mirrored in the legislation of the Australian Capital Territory, New South Wales, Northern Territory, Tasmania and Victoria. At present Queensland chooses to retain the former system while Western Australia and South Australia are currently moving toward the UEL system. These statutes were not meant to eliminate all of the preceding common law, but they override much of it. All statutory provisions mentioned here are from the Commonwealth’s model Act, and should be identical in all State and Territory Acts. A separate chapter may be written in future to address Queensland's evidence law.
This chapter has been written with circumstantial investigators and other people performing investigative functions in mind. For that reason rules regarding the questioning of witnesses at trial are not addressed here. The point is to avoid errors and wasted time in sourcing evidence to be presented at court. It is essential that when evidence is obtained that it be done in a manner which does not transgress the rules, and that the sources of evidence relied upon be acceptable. Having to obtain evidence anew may result in an opportunity for witnesses to re-think and collude with others, tainting their reliability. It may even result in refusal on the part of witnesses to devote further time. An investigator therefore needs to know the rules of evidence as a tool of trade, even if acting under detailed instructions from someone else who already knows.