For fully managed (similar to Heroku Postgres) Crunchy Bridge [1] supports replicas across regions including say EU to AU. Which would actually couple really well with Fly.io for the app side.
The simple solution is to run Postgres replicas in those regions and do local reads for your app while directing the writes to the master region. This works well for read-heavy apps, and you can also put the master in a geographically central location to help with write latency.
If you need cross-region multi-master then I recommend something like CockroachDB or Yugabyte that have regional distribution as a core feature.
Heroku is only available in EU and US east. Latency from Australia is somewhat unavoidable. You can maybe reduce it by using Cloudflare or similar to terminate TLS as close to your users as possible.
Cloudflare give different Anycast IPs for each plan and they don't always hit the local ingestion point. Some people shot themselves in the foot with free/pro plan in certain markets (India, aus etc..) I don't believe in May 2021 local ingestion (TLS termination) is happening on anything less than Business plan in MEL or SYD
Currently running my app + db on Heroku in the EU and would like to scale out since latency is abysmal for users in Australia and Asia.