Geoff,
Take a look at Tayda. You can get new switches with tall (up to 9.5mm) buttons for $0.04 USD.
It may be easier/cheaper to replace the actual switches.
http://www.taydaelectronics.com/electromechanical/switches-key-pad.html
They ship to Australia.
I've ordered from Tayda several times. They are great for small orders.
I've ordered shift registers, LEDs, caps, resistors, temp ICs, I2c clock chips, resonators, crystals, etc....
Their prices are great for what they have and their shipping is really inexpensive as well.
Takes anywhere from 1-3 weeks for delivery, with most of my orders getting
here to the USA in about 10-12 days. 1 order came in less than 1 week.
--- bill