Australia Definitive

100 Cents = 1 Dollar

10 Cents

Years: 1966-1984
Metal: Copper-nickel
Weight: 5.65 gm
Diameter: 23.6 mm
Thickness: 2 mm
Engravers: Arnold Machin (obverse)
                Stuart Devlin (reverse)

This coin is of 1974 and bears a 2nd portrait of Queen Elizabeth II [young].

Obverse design shows the young bust of Queen Elizabeth II with tiara right.
Reverse design show a Lyrebird in a courtship display along with the denomination.

Lyrebirds are among Australia's best-known native birds. They are most notable for their superb ability to mimic natural and artificial sounds from their environment. Lyrebirds are notable because of the striking beauty of the male bird's huge tail when it is fanned out in display; and also because of their courtship display.

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20 Cents

 Years: 1966-1984
Metal: Copper-nickel
Weight: 11.3 g
Diameter: 28.5 mm
Thickness: 2.5 mm
Engravers: Arnold Machin (obverse)
                Stuart Devlin (reverse)

This coin is of 1981 and bears a 2nd portrait of Queen Elizabeth II [young].

Obverse design shows the young bust of Queen Elizabeth II with tiara right.
Reverse design show a Duckbill Platypus along with the denomination.

Duckbill Platypus

Platypus is found mainly in Eastern Australia including Tasmania. It is the only mammals that lay eggs instead of giving birth to live young.It is one of the few venomous (poisonous) mammals, the male platypus having a spur on the hind foot that delivers a venom (poison) capable of causing severe pain to humans. It has unusual appearance because of its duck-billed, beaver-tailed, otter-footed structure.The platypus is the animal emblem of the state of New South Wales.

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1 Dollar 

 Years: 1985-1998
Metal: Aluminium-bronze (Cu 92%, Al 6%, Ni 2%)
Weight: 9.0 g
Diameter: 25 mm
Thickness: 2.5 mm
Engravers: Raphael David Maklouf (obverse)
           Stuart Devlin (reverse)

 This coin is of 1998 and bears  3rd portrait of Queen Elizabeth II.

Obverse design shows Head of Queen Elizabeth II (3rd portrait).
Reverse design shows "Mob of Roos" 5 kangaroos along with the denomination.

Kangaroos are often colloquially referred to as 'roos'. Male kangaroos are called bucks, boomers, jacks, or old men; females are does, flyers, or jills, and the young ones are joeys. The collective noun for kangaroos is a mob, troop, or court. Mobs usually have ten or more kangaroos in them. Living in mobs provides protection for some of the weaker members of the group.

A common myth about the kangaroo's English name is that "kangaroo" was a Guugu Yimithirr phrase for "I don't understand you." According to this legend, Lieutenant Cook and naturalist Sir Joseph Banks were exploring the area when they happened upon the animal. They asked a nearby local what the creatures were called. The local responded "Kangaroo", meaning "I don't understand you", which Cook took to be the name of the creature. The Kangaroo myth was debunked in the 1970s by linguist John B. Haviland in his research with the Guugu Yimithirr people. 

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Here are the all 3 coins to get the idea of their comparative sizes.