Graph seems to be comparing 6 months of 2018 with full years before that which would vastly overstate the decline.
If anybody is interested in the actual data, I think the data is pulled from "NSW HIV Surveillance Data Reports": https://www.health.nsw.gov.au/endinghiv/Pages/tools-and-data...
The trend seems to be downward, but it's more like ~10% in the past couple years than a step change.