It was never 5 AUD.
It's 5 Pounds, Euros or USD but not AUD.
On an article from the 16th of November, 2013, it was stated that Pokebank would cost $6.50 AUD from the start.
>A yearly fee to Pokémon Bank will cost AU$6.50 and NZ$8.50. Nintendo has even attached a statement to the pricing to explain why Pokémon Bank is a paid subscription service:
Source
In the case where it increases in the future, it'll probably be because of one or more of the following reasons;
1. Inflation (around 2.5% a year on average in Australia)
2. Weakening Australian Dollar. They could adjust the price to match the Yen or something in terms of currency conversion - the AUD is quite weak right now, especially against the USD
3. Loss in profit. They'd obviously crank up the price if they needed to make a little more money to break even or something.
