Due to a high number of requests, we have changed the settings for how the Invoice Number is listed for some of our users. Accounts that have "British Pound" selected as their currency will now see: Tax Invoice #.
"Tax invoice is standard format invoice required under GST system. ... A GST-registered company must have a valid Tax invoice from the supplier in order to claim back the GST they have paid on the purchase for their business. A tax invoice is the primary evidence to support an Input Tax Credit claim". More can be read about this here.