You have to remember that that Ben and Olivia disappeared from a place where almost everyone were visitors. Those who were there on the night in question were long gone by the time this came to trial, many of them from other parts of New Zealand, and even overseas. Tracking down witnesses would have been a very difficult task for the defence.
Also, unlike places like the USA and Canada, NZ's pre-trial discovery rules are appallingly slack. It would not be overstating the case to say that the Police are not under any obligation to reveal to the defence anything they have in their possession, even exculpatory evidence. For example, in the Arthur Thomas case, the police had a witness who lived near the Crewe farm who heard gunshots ring out in the early evening at a time when Thomas had a clear alibi. The Police suppressed that evidence through both trials, and consequently, the witness (Julie Priest) was never called to testify at either trial. In the USA, that is exculpatory, and if the prosecution failed to reveal this at discovery, not only it would be grounds for immediate reversal of any conviction, but the prosecutor who failed to reveal it in discovery would very likely find himself charged with Prosecutorial Misconduct.
That witness was found by David Yallop ("Beyond Reasonable Doubt") and was one of the key pieces of evidence that got the Thomas conviction quashed.