Many of our older villas and bungalows have ceramic or earthenware pipes underground. Heavy chemicals generate heat. If the blockage is 100% solid, the chemical heat can crack old pipes. If the plunger and baking soda don't work, call a local plumber (yes, it hurts the wallet, but it saves digging up the driveway).
If you prefer to avoid harsh chemicals, several effective DIY methods are recommended by NZ drainage specialists : What Cleaning Products Can You Use with A Septic Tank best drain unblocker nz
: A powerful gel formula that works fast. It's available in various retail stores across New Zealand. Many of our older villas and bungalows have