Data: National Notifiable Diseases Surveillance System (NNDSS)
You can access data here: Link
According to the NNDSS website: "The National Notifiable Diseases Surveillance System (NNDSS) was established in 1990 under the auspices of the Communicable Diseases Network Australia. The System co-ordinates the national surveillance of more than 50 communicable diseases or disease groups. Under this scheme, notifications are made to the States or Territory health authority under the provisions of the public health legislation in their jurisdiction. Computerised, de-identified unit records of notifications are supplied to the Australian Government Department of Health on a daily basis, for collation, analysis and publication on the Internet, (updated daily), and in the quarterly journal Communicable Diseases Intelligence.
Notification data provided include a unique record reference number, state or territory identifier, disease code, date of onset, date of notification to the relevant health authority, sex, age, Indigenous status and postcode of residence.
The quality and completeness of data compiled in the National Notifiable Diseases Surveillance System are influenced by various factors. Notifications may be required from treating clinicians, diagnostic laboratories or hospitals. In addition, the mechanism of notification varies between States and Territories and in some cases different diseases are notifiable by different mechanisms. The proportion of cases seen by health care providers which are the subject of notification to health authorities is not known with certainty for any disease, and may vary among diseases, between jurisdictions and over time."
I have only included two key diseases as the number for the other cases were relatively small. You can use the link above to access a more detailed dataset.
Overall, in 2020, and especially during the national lockdown in Australia, between March and May, the number of notifications for sexually transmissible infections declined. The second decline between July and September can be due to the more strict lockdown in Melbourne/Victoria.
The next figure shows the monthly cumulative pattern which better presents the overall decline in the number of notifications.