Which is why, coming the other way, i.e. TO NZ and Aus, people who have been on farms or who have visible organic material on their footwear, are asked to remove it - and give up any in their luggage - for steam cleaning, rather than risk new organisms entering the countries. I guess fungus spores would be pretty much invisible to the naked eye of a quarantine officer though, especially if it was just a little bit of material underneath or in the treads.