I don't know exactly what Australia uses/mandates but since this was not tagged with a country...
It really depends on the bag and the environment in ends up in, as well as the time-frame considered. An EU document on the matter from 2019 says/summarizes:
The European Union has recently banned the use of oxo-degradable plastics because of a
lack of consistent evidence about speed of breakdown in the environment, and fears that
false claims around this are misleading consumers. The directive 2019/904 came into force
on 3 July 2019 and has to be transposed by the Member States by 3 July 2021. In addition,
there are concerns that the ‘biodegradable’ label could lead to more littering, as consumers are more relaxed about discarding bags, rather than reusing them, assuming they will break down and pose no threat to the environment.
Five different types of plastic bag available from UK high-street retailers at point of sale were used in this study; this included two types of oxo-degradable bag (Oxobio1 and
Oxobio2), one biodegradable bag, one compostable bag and one high-density polyethylene
(HDPE) carrier bag. [...]
The biodegradable, oxo-degradable and conventional plastic bags all stayed functional in the
marine environment and soil for the three years of the study and could hold 2 kilograms of
groceries in them without breaking. The researchers highlight that the compostable bag
remained intact in soil but was the only type of bag that completely disappeared from the
marine pouch within three months. In air, the bags became brittle and formed smaller
particles visible to the naked eye. Collectively, the results showed that none of the bags
could be relied upon to show any substantial breakdown over a three-year period across all
of the environments. The oxo-degradable or biodegradable formulations do not show
sufficiently advanced rates of breakdown to have a clear advantage in terms of reducing
marine litter compared to conventional bags.
- Napper, I. and
Thompson, R. (2019).
of Biodegradable, Oxobiodegradable,
Conventional Plastic Carrier
Bags in the Sea, Soil, and
Open-Air Over a 3-Year
Science & Technology,
53(9), pp.4775-4783. https://doi.org/10.1021/acs.est.8b06984
In essence, the result of the paper cited is summarized in this graph:
The types of bags tested are detailed in the following table, which alas doesn't get to some detailed chemical composition and just relies on (UK) labeling (click to zoom):
Basically: oxobio1 = D2W logo, oxobio2 = EPI logo, biodegradable = exoplastics logo (ISO 14855), compostable "recyle me with food" EN13432, conventional (HDPE).