It is possible for a trial to be fair without a central witness giving evidence in person and being cross-examined. The witness's evidence may be read at trial but the fact-finder may still have adequate means of testing the reliability of that evidence.
A fair trial is one where the law is accurately applied to facts that are determined impartially. Impartiality can exist when an unbiased fact-finder uses adequate means to assess the reliability and weight of the evidence.
It might be that there are corroborative witnesses who do give evidence and who can be cross-examined. There might also be a similarity between the evidence of independent complainants that is so unlikely to be coincidence that their mutual reliability is virtually assured. In such cases, where the defence can cross-examine the witnesses who support the absent witness, there may be found to be sufficient factors to counter-balance the absence of the central witness so that the defendant is not deprived of a fair trial.
But in other cases the absence of the central witness may not be counter-balanced. There may be no corroborative oral testimony. There may be no evidence that the defence could call to contradict the absent witness. In such cases the fact-finder may be unable to impartially assess the reliability of the absent witness, there being no one for the defence to cross-examine on the central issues.
The rule against hearsay, the exceptions to that rule, and the rule excluding evidence when its probative value is outweighed by the risk of improper prejudice to the defence, are the means by which the common law has endeavoured to ensure the fairness of trials when witnesses are not available for cross-examination. Often these rules have become statutory.
Over the last few years a storm gathered in Europe over this. The European Court of Human Rights had developed a rule that a conviction could not be based on the evidence of a witness who could not be cross-examined if the evidence of that witness was central to the prosecution case in the sense of being the sole evidence against the defendant or of being decisive evidence against him: Unterpertinger v Austria judgment, 24 November 1986, § 33, Series A no. 110. The UK Supreme Court criticised this rule in R v Horncastle  UKSC 14 (noted here as an update to the entry on Al-Khawaja and Tahery v R  ECHR 110, 27 January 2009), and only the most obtuse reader would fail to see that if the Grand Chamber did not allow the UK courts to continue to apply the discretionary approach rather than the Strasbourg rule, continued participation of the UK in European criminal law would be unlikely.
So inevitably Strasbourg yielded and departed from its rule. On appeal from the Chamber decision in Al-Khawaja and Tahery, the Grand Chamber held 15-2 that the rule did not apply where the law of a State contained sufficient safeguards:  ECHR 2127 (15 December 2011).
The majority held that the underlying principle is that the defendant in a criminal trial should have an effective opportunity to challenge the evidence against him (127). The question was whether there were sufficient safeguards to secure the defendant's right to a fair trial (130).
"Also, in cases concerning the withholding of evidence from the defence in order to protect police sources, the Court has left it to the domestic courts to decide whether the rights of the defence should cede to the public interest and has confined itself to verifying whether the procedures followed by the judicial authorities sufficiently counterbalance the limitations on the defence with appropriate safeguards. The fact that certain evidence was not made available to the defence was not considered automatically to lead to a violation of Article 6 § 1 (see, for example,Rowe and Davis v. the United Kingdom [GC], no. 28901/95, ECHR 2000 II). Similarly, in the case of Salduz, cited above, § 50, the Court reiterated that the right to legal assistance, set out in Article 6 § 3 (c) was one element, amongst others, of the concept of a fair trial in criminal proceedings contained in Article 6 § 1." "While we understand the nature of the challenges faced by the prosecution when key witnesses die or refuse to appear at trial out of genuine fear, the protections guaranteed by Article 6 speak only to the rights of the defence, not to the plight of witnesses or the prosecution. The task of this Court is to protect the accused precisely when the Government limit rights under the Convention in order to bolster the State’s own position at trial. Counterbalancing procedures may, when strictly necessary, allow the Government flexibility in satisfying the demands of Article 6 § 3 (d). Our evolving application of the sole or decisive test, however, shows that this exception to the general requirement of confrontation is not itself without limits in principle. In the end, it is the job of the Government to support their case with non-hearsay corroborating evidence. Failure to do so leaves the Government open to serious questions about the adequacy of their procedures and violates the State’s obligations under Article 6 § 1 in conjunction with Article 6 § 3 (d)."
And their concluding quotation was from a New Zealand case, R v Hughes  2 NZLR 129 (CA) at 148-149:
"We would be on a slippery slope as a society if on a supposed balancing of the interests of the State against those of the individual accused the Courts were by judicial rule to allow limitations on the defence in raising matters properly relevant to an issue in the trial. Today the claim is that the name of the witness need not be given: tomorrow, and by the same logic, it will be that the risk of physical identification of the witness must be eliminated in the interests of justice in the detection and prosecution of crime, either by allowing the witness to testify with anonymity, for example from behind a screen, in which case his demeanour could not be observed, or by removing the accused from the Court, or both. The right to confront an adverse witness is basic to any civilised notion of a fair trial. That must include the right for the defence to ascertain the true identity of an accuser where questions of credibility are in issue."
Although statute has permitted the limitation of the right of a defendant to know the identity of a witness in certain limited circumstances, the right to a fair trial remains absolute in New Zealand, as no doubt it does in the United Kingdom.