I saw a post around Christmas time regarding what measures should be taken to help prevent the spread of didymo.
Biosecurity New Zealand have now got quite a bit of information on this.
http://www.biosecurity.govt.nz/pests-diseases/plants/didymo/
I also noted that DoC have stopped swimming and diving at of Te Waikoropupu Springs in Golden Bay to prevent its spread.
cheers,
Shane