The notion of the validity period is intended to prevent a situation where a reference number is issued concerning the licence of a purchaser where the purchaser, for whatever reason, no longer retains that licence eligibility, for example, if a person has had their licence rescinded or if they are subject to a prohibition order, or some such thing.
If a reference number were valid ad infinitum, there is a chance the licence could be suspended, revoked, or whatever during the period that the reference number was purporting to cover as being valid. Firearms regulations must be tabled in Parliament and available for scrutiny by committees of both Houses. By prescribing in regulation the validity period, the scheme ensures that a person with a reference number can transfer the sale of a firearm only within the reasonable period that the check covers the licence as being valid.
If we remember that there is no information being kept about the firearms involved in relation to the reference number—it is simply about the vendor and the purchaser—the reference number provides the vendor with proof that they have done their due diligence. However, that would have to be subject to some time limitation, lest we run the risk that the validity of the purchaser's licence changes, or expires, in the interim.