Press Release – Canterbury District Health Board Canterbury DHBs Community and Public Health unit has issued a health warning for the Cust Main Drain at Skewbridge. The warning follows a high cover of potentially toxic algae (benthic cyanobacteria), as well as detaching mats, being found in the … Canterbury DHB’s Community and Public Health unit has issued a health warning for the Cust Main Drain at Skewbridge. The warning follows a high cover of potentially toxic algae (benthic cyanobacteria), as well as detaching mats, being found in the Cust Main Drain. People should avoid the area and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.