Once established there is no known way of removing the invasive algae.
It has already impacted anglers, jet boaters, tourist operators and hydro electricity generation, says Rotorua-based DOC ranger Manu Rangiheure.
"Although didymo is not considered a significant human health risk, the algae poses a huge risk to freshwater fisheries. Freshwater fish including trout, salmon, native galaxiids (such as whitebait) and bullies are most likely to be affected. Didymo also has the potential to alter the invertebrate communities on which fish feed."
Classed as an Unwanted Organism under the Biosecurity Act, it is an offence to knowingly spread it and a conviction for doing so could incur five years imprisonment, and/or a fine of up to $100,000.
But the real risk is spreading the algae accidentally.
People do not need to worry if they spend a day on a single lake or river,
But they should always check, clean and dry, when moving between two or more waterways.
For example, if trampers were to walk up a stream be all day that would be fine.
But if they intend to cross a ridge, then walk down another stream bed, they should clean their boots first.
Every place didymo has been found in the South Island has been a human access point to a waterway, but it's thought the algae isn't spread by birds.
Summer advocacy rangers will be out and about to mobilise public awareness.
DOC has joined forces with regional councils and iwi to create the summer advocacy teams.
They will patrol lakes and rivers, reminding users of the risks associated with moving gear between rivers and lakes.
"It's not just boats and fishing equipment that can spread this species. Bikes, wetsuits and even hiking boots are also a vector.
"People visiting the rohe from other parts of New Zealand where didymo has been confirmed are my main concern. Didymo is microscopic so it can be spread by just a single drop of water hiding on the bottom of a tramping boot. It's often referred to as a biological hitchhiker."
The algae was first discovered in New Zealand in 2004, with the first recorded finding in the southern hemisphere at Waiau River, Southland.
DNA suggested it was introduced from North America on fishing equipment.
While the spread of didymo throughout the South Island was initially rapid, it seems to have slowed.
And as of October there have been no confirmed sightings in the North Island.
Check Clean Dry
Wherever possible, restrict the use of equipment to a single waterway.
This reduces the chance of spreading aquatic pests (not just didymo) between waterways.
Where there is a need to use multiple areas of water, follow check, clean, dry protocol as recommended by the Ministry for Primary Industries.
1) Check:
Before leaving the river, remove all obvious clumps of algae and look for hidden clumps. Leave them at the site. If you find clumps later don't wash them down the drain, treat them with the approved methods below, dry them and put them in a rubbish bin.
2) Clean:
Soak and scrub all items for at least one minute in either hot (60°C) water, a 2% solution of household bleach or a 5% solution of salt, antiseptic hand cleaner or dishwashing detergent.
3) Dry:
If cleaning is not practical (livestock, pets), after the item is completely dry wait an additional 48 hours before contact or use in any other waterway.
