You're not wrong. Similar concept to people applying 4 years and 8 months after gaining residence (provided they don't travel) the first 8 months in the first year of the 5 year period counted backwards, the person's visa status likely isn't resident (maybe work visa, or no visa/not yet in NZ).
If you've been granted entry permission as an Australian, then effectively you are a resident on the days you are in NZ.
Only your visa expires when leaving, but your 'resident' status doesn't go away. To demonstrate the concept, people outside NZ with expired travel conditions can apply for PR too as long as they meet the PR requirements... it doesn't mean that they left NZ and their RV travel conditions expired that all those days they spent in NZ as residents are "gone".
If in doubt, you can ask INZ to count the days you're here