Decimal currency was introduced in Australia in 1966. According to the Reserve Bank's inflation calculator (see http://www.rba.gov.au/calculator/), $1 in 1966 would have bought goods and services worth $11.51 in 2011. So 5¢ today has the value of less than half a cent in 1966.
I can't say, however, that I'm seriously troubled by the continued existence of 5¢ coins. Yes, they do accumulate, and many coin-operated machines won't take them, but personally I seem usually to be able to "move them on".
What would be of greater utility would be to down-size all the silver coins, as New Zealand has done. Perhaps the opportunity could also be taken to adjust the sizes of the $1 and $2 coins, so that the $2 coin was larger than the $1 coin. Although a direct "swap" would obviously be impossible, more logical designs of these coins would not seem hard to achieve.