By comparing the product prices between 4 chemists/pharmacies, Chemist Warehouse is the cheapest. It is $3.25 (33 %) cheaper than next price leader (Priceline Pharmacy).
The current price at $6.49 is $3.25 (30 %) more than the historical cheapest price of $4.99 recorded on 16 May 2018.
The price discount recorded on 20 Apr 2018 is part of Chemist Warehouse's "Mum... The Only Hero" promotion campaign which start from 20 Apr 2018 to 13 May 2018
You may want to consider similar and cheaper product Colgate Optic White Express White 85g can be found at ebay Australia at $4.99.