No refund for flight cancelled by jetstar

My jetstar flight was cancelled in Jul 2011 due to the ashclould.

At the time I asked for a refund and was told a credit would be on my flight for the next time I fly.

I then find out that they had issued a voucher with an expiry date in 3 months which I did not receive.

This in my mind is theft - you pay for a flight and get nothing. They say because they issued a voucher that has expired there is nothing they can do.

Absolute robbing B******