Incorrect payment gateway used

If a user makes a purchase using say AUD and chooses to pay by bank transfer.
The order is saved and pending payment in AUD.
The user can then choose to pay for the order and will visit the checkout page where the order is shown in AUD.
If they then change currency to say CAD, the order remains in AUD, but the available payment gateways change to those enabled for CAD, even though they order is in AUD.
This allows an AUD payment to be processed by the wrong payment gateway.
The plugin should prevent changing to another currency in this situation.