The Tenants NSW website says this: “notice can be given on or before the last day of the fixed term” and “Give a minimum 14-day termination notice. The termination date in the notice can be the last day of the fixed term or up to 14 days after. You have to give this notice before your fixed-term agreement ends. Vacate by the date in your notice.”
In other words, if the tenant gives notice on or before the last day of their tenancy agreement, then it only has to be 14 days from the date they give the notice. If they wait until after the lease has expired, it’s 21 days.