The new Parliament House opened in 1927 and this florin, with a special reverse side, commemorated the occasion. Like other Australian florins of the time (see this link for them), the Parliament House florin contains 0.336 troy ounces of silver.

Approximate catalog values are:

worn: $10 US dollars approximate catalog value
average circulated: $15
well preserved: $20
fully uncirculated: $85

Our Important Terminology page presents concepts for converting these catalog values to actual buy and sell values.

In the case of Lean Thomas's coin, which has uneven toning, the actual value will probably be affected negatively. Collectors do not like coins that are toned in one spot and not in another.