Edit: I have been pointed to the decision of Connex Group Australia v Butt  NSW 379 on this point. White J delivers a comprehensive judgment explaining his decision to allow lay opinion about the substance of conversation, where the words themselves could no longer be recalled.
I’ll write a full post on it when time permits.
78. Exception – lay opinions
The opinion rule does not apply to evidence of an opinion expressed by a person if-
(a) the opinion is based on what the person saw, heard or otherwise perceived about a matter or event; and
(b) evidence of the opinion is necessary to obtain an adequate account or understanding of the person’s perception of the matter or event.
‘Necessary’ in the context of (b) does not refer to absolute necessity: Jackson v Lithgow City Council  NSWCA 136. Allsop P noted what had been written in Odgers Uniform Evidence Law (2008, 8th Ed, Lawbook Co, p 302) prior to the matter going to the High Court, but found that (unlike as is suggested there) no distinction needs to be drawn between an opinion which is only a ‘compendious description’ of what was perceived and an opinion which ‘actually draws an inference’ from what was perceived.
Basten JA (agreeing with Allsop P and Grove J) said [at 71],
71 When used in the Evidence Act, the term “necessary” connotes a higher hurdle to surmount than that which is ‘helpful’, ‘convenient’ or ‘desirable’, but does not require absolute necessity, in the sense of being the sole means of proof. Whether the exception is satisfied in a particular case may need to take account of the purpose or purposes underlying the general exclusion and the purpose of the exception.
The Court did not consider it significant to admissibility that the evidence would have been easily proved by other means (calling one of the officers) or that the foundation for the opinion was ambiguous. Basten JA outlined in some detail judicial authority concerning what ‘necessary’ may mean in a variety of curial contexts.
The facts in this case were quite unusual. They are set out in Jackson v Lithgow City Council  NSWCA 312. The appellant, originally the plaintiff in a negligence action, was found lying unconscious in a concrete drain in a park in Lithgow shortly before 7 am on 18 July 2002 with serious injuries, cuts and abrasions. He had taken his dogs for a walk at about 3:30 that morning whilst intoxicated. He had no memory of the events in question or of events from the middle of the day before. No one saw the accident. There was no direct evidence as to the position of the appellant’s body in the drain when he was found. The appellant sued the local council having care and management of the park, alleging that he had fallen over the low, unfenced retaining wall of the drain and down approximately 1.5 metres on to the concrete drain.
The primary judge found in favour of the respondent council. The respondent did owe someone in the position of the appellant, walking in the park at night, a duty to exercise reasonable care for his or her safety and that such duty was breached by the respondent failing to take steps to avoid the risk of foreseeable injury to someone falling over the wall at night. However, the primary judge concluded that the appellant had not proved how he fell and came to be injured, the evidence not permitting a conclusion that the appellant had stumbled over the low wall and fallen down on to the concrete, or that the appellant had approached the drain when it was dark.
The Court of Appeal found in favour of the plaintiff, relying upon a ‘retrieval record’ (a note made by ambulance officers who attended upon the plaintiff and took him to hospital). Unusually, neither ambulance officer had been called as a witness, but their notes had been tendered without objection. It then became the subject of some discussion as to whether the notes were admitted under s 78 (lay opinion), s 69 (business records), or some exception to the hearsay rule.
The Lithgow City Council took the matter to the High Court because part of the evidence placed before the Court of Appeal had been incorrectly transcribed. When remitted to the Court of Appeal before the identically-composed Court the result was again judgment in favour of the plaintiff, and the findings expressed above.