Definition from the Food Standards website-
What is a best-before date?
A best-before date is the last date on which a food can be expected to retain all of its quality attributes, provided that it has been stored in accordance with any stated storage conditions.
Quality attributes include things such as colour, taste, texture and flavour, as well as any specific qualities for which express or implied claims have been made. For example, the freshness of the food or certain vitamin levels.
A food that has passed its best-before date may still be perfectly safe to consume, but its quality may have diminished.
Food date-marked with a best-before date can be sold after this date, provided the food is not damaged, deteriorated or perished. It is an offence under New Zealand and Australian State and Territory Food Acts to sell food that is damaged, deteriorated or perished at any time, regardless of whether the food is within its specified date mark or not.