Princetane is correct; gold-plating an Australian coin is a crime in Australia. And you don't have to refer to arcane British laws, either. Specifically, the Crimes (Currency) Act 1981
, article 16:
16 Defacing or destroying current coins or current paper money
A person shall not, without the consent, in writing, of an authorized person, intentionally deface, disfigure, mutilate or destroy any coin or paper money that is lawfully current in Australia.
(a) in the case of a person, not being a body corporateŚ$5,000 or imprisonment for 2 years, or both; or
(b) in the case of a person, being a body corporateŚ$10,000.
And in case one might think there's some ambiguity whether a plated coin is "defaced", Article 3(4) specifically states:
(4) Without limiting the meaning of the expression "deface", a coin or paper money shall be taken to have been defaced for the purposes of this Act if some or all of its surfaces are coated with any material.
This covers everything, from Macquarie Mint gold-plating a normal coin for profit, to someone trying to make a 10 cent piece look like a dollar coin by painting it yellow, to a company placing an advertising sticker on a coin and putting it back in circulation. Technically, even coating a coin in varnish or oil might qualify as "defacement" under this legislation, though there might be some legal precedent I'm unaware of for defining "coating" that implies permanence.
You'll also notice "intentionally" in the wording. It's not a crime if you accidentally drop a coin into a bucket of paint, fish it out and let it dry. Or if you drop a coin into a chlorinated pool so that it turns strange colours after a few days - which, given we've seen no pictures from the OP, is entirely possible what has happened in this instance.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis