That is actually described in some detail in the documentation:
PostgreSQL allows you to specify time zones in three different forms:
A full time zone name, for example America/New_York. [...]
A time zone abbreviation, for example PST. [...]
In addition to the timezone names and abbreviations, PostgreSQL will accept POSIX-style time zone specifications of the form STDoffset or STDoffsetDST, where STD is a zone abbreviation, offset is a numeric offset in hours west from UTC, and DST is an optional daylight-savings zone abbreviation, assumed to stand for one hour ahead of the given offset. [...]
[...]
Another issue to keep in mind is that in POSIX time zone names, positive offsets are used for locations west of Greenwich. Everywhere else, PostgreSQL follows the ISO-8601 convention that positive timezone offsets are east of Greenwich.
So the POSIX equivalent for AEST would be -10 or UTC-10.
