When we were entering the cost including GST, this is where we were experiencing the issue. Since looking into it a bit more yesterday, we realised it may be a possible rounding issue? Turned out the cost was meant to be 0.297, not rounded up to 0.30 before enrty, which the person who had filled the form in had done before sending to us.
But when entered into acrobat, this will round up itself and then give us the correct calculation.
Not sure if anything can be done about the rounding, but now that we know, we can work around this if need be, will just have to keep an eye out and let everyone know not to round anything up!