"Crews are donning level three splash suits to put out the chemical reaction," the fire service spokesman said earlier this morning.
Firefighters are now being decontaminated.
A Countdown spokesperson said the fire was in one of the outdoor bins at the Warkworth store.
"A damaged box of pool chemicals, which we sell, was placed in the bin and has reacted to something. We will investigate this further to ensure it doesn't happen again.
"No one was hurt. This has now been put out by local fire fighters and the
store has reopened.
Three fire trucks from Warkworth, a hazardous materials unit and support crews were at the scene.
Auckland Council's pollution line has been called because there's concern run-off will go into drains.
Neville St is closed.