The IANA maintains the Olson database. The question of what timezone abbreviation(s) should be used for Australia was discussed in the IANA's tz mailing list here (the discussion spanned two months: March 2013, April 2013).
There seems to be strong opinion on all sides as to what the abbreviations should be and those strong opinions have resulted in gridlock.
Some say the abbreviations are a relic of the past and should not be used and the ambiguity should not be fixed to help discourage its use.
Apparently there is no recognized authority in Australia defining the abbreviations. Some say conflicting organizations use different timezone abbreviations, and so as not to pick political sides, the IANA chose EST for both standard and daylight savings times.
For now, the Olson DB uses EST for all timezones for all dates in Australia/Sydney:
In [60]: import pytz
In [61]: sydney = pytz.timezone('Australia/Sydney')
In [68]: [(date, tzabbrev) for date, (utcoffset, dstoffset, tzabbrev) in zip(sydney._utc_transition_times, sydney._transition_info)]
Out[68]:
[(datetime.datetime(1, 1, 1, 0, 0), 'EST'),
(datetime.datetime(1916, 12, 31, 14, 1), 'EST'),
(datetime.datetime(1917, 3, 24, 15, 0), 'EST'),
(datetime.datetime(1941, 12, 31, 16, 0), 'EST'),
(datetime.datetime(1942, 3, 28, 15, 0), 'EST'),
(datetime.datetime(1942, 9, 26, 16, 0), 'EST'),
(datetime.datetime(1943, 3, 27, 15, 0), 'EST'),
...]
In [69]: set([tzabbrev for utcoffset, dstoffset, tzabbrev in sydney._transition_info])
Out[69]: {'EST'}
This shows that in the Australia/Sydney timezone, EST is used across every transition boundary.