We've taken our bikes to Australia 3 times. We've always given them a good clean in the hotel bathtub (and left a big tip for the maid) and have never had any problems with MAF. They just had a look at them to check they were clean. At least last time (I can't remember for sure about the other times) I declared that I had ridden through forests and farmland - I did have slicks on the bikes though and had only ridden on the road - they might be more fastidious if you had been offroad. They were fussier about my tent though - taking it into a sealed room to check (it was also fine)