Courier Service Selection order value to take into account currency

By default anything weighing 2kg+ goes via DPD/DHL/UPS etc however our client also wants anything that's small and light but also of high value to go via courier instead of Royal Mail.

I went to change the courier service selection for that client so that anything worth over £50 goes via courier regardless of weight. However the field simply is based on order value regardless of currency. As a result an order worth nearly $AUD60 is actually worth £34 and is being sent courier when it shouldn't.