Apparently however small innocuous packets/letters don't receive quite as much screening, as I've heard of people getting marijuana seeds in the mail just fine.
It's far more difficult than drug smuggling which only works in some reasonable quantity - and becomes more like information smuggling in biological form where you only need one copy.
"The precedent-setting agreement classifies a spacecraft as an export. Once launched, satellite components were judged to be “severed from the possibility of trade within the United States” and therefore considered to be exports, according to Customs officials."
If you are interested in bird conservation in NZ, please visit Karori park in Wellington and contribute.
BTW: some parks in US also have installed brushes to wipe your boots for preventing the transmission of invasive species seeds. I think there are a few in Wisconsin that I remember.