Value Of Metal Coated Australian Pennies

G'day & welcome to the CCForum,
I've never heard of that being done, except by someone trying to make "jewelry".
I have seen gold-plated pennies, for use in "two-up" (a form of gambling).
I have seen pennies pressed into the shape of a slouch hat (as worn by Australian soldiers), and gilded, to give them a gold-like appearance. These were mounted as key-rings.
The Post Office sold silver-plated pennies a few years ago, mounted for wear as a pendant.
I suppose nickel-plated pennies would have a silver-like appearance ?
I expect most collectors would consider any form of plating would reduce a coin's value to zero.
As for the coating acting as a preservative: bronze coins survive quite well as long as they're kept dry.
Peter in Oz

The main problem with a replating (besides it looking un-natural) is you can't really tell how well preserved the coin is underneath it. Certainly if the plating is the wrong kind of metal, a galvanic reaction can cause the coin to corrode even faster than an unplated coin.

The other use for plated pennies like this was to be attached to souvenir keyrings. I've seen the keyrings for sale in those tacky "Ocker" souvenir shops. And I've seen the plated coins themselves in coin dealer scratchtrays - normally with a huge torn hole in the plating where the coin had been ripped off of the keyring it had been glued onto.