However, Mr Fennessey said the unwanted organism was a Southern problem and as far as he was aware there was no didymo in the North Island to spread.
"It is okay to tramp and kayak with felt-soled shoes but not fish," he said.
"More and more people are fishing, there is a resurgence of trout fishing and the reality is that when you step into the water with everything flowing downstream and making the rocks slimy, you take your life into your own hands."
On January 1, Gary Griffiths, wearing regular boots, slipped on a rock and badly broke his tibia and a bone in his ankle while fishing in the Tukituki River.
He had to walk about one hour on his broken leg to safety, which was "bloody painful", he said.
The trout fisherman of about five years said the felt-soled shoes could have prevented the slip and were missed by most anglers.
"The guys think they are great ... it is like having sandpaper on your soles that grip to the rocks."
Mr Fennessey said more angling accidents would happen as fishermen wore studded boots or sneakers as replacements.
The Ministry for Primary Industries stipulates that it is an offence to knowingly spread an unwanted organism, with penalties of up to five years' imprisonment and/or a fine of up to $100,000.
There are no current treatment options to remove or control didymo in waterways, with the Government employing a public awareness campaign to encourage freshwater users to check, clean and dry equipment when moving between rivers.