As per @SaintCat paying via BPay you just pay an amount over...

As per @SaintCat paying via BPay you just pay an amount over your rights entitlement amount. Any amount over your rights entitlement will be put to subscribing for additional shares under the shortfall facility.

Say you have to pay $1,000 for your rights entitlement but pay $2,000 instead then you would get 250,000 shares as per your entitlement plus a further 250,000 shares via the shortfall facility assuming that the rights issue is not over-subscribed.

If the rights issue is over subscribed then either the shortfall applications would be scaled back (on an equitable basis according to the booklet) or the company may choose to accept over subscriptions (although I don't recall any mention of this so wouldn't expect that to happen).