Three fire crews responded to a waterfront blaze in the Sydney suburb of Pyrmont on Friday afternoon.
Bystanders called triple zero when the blaze began just after 3.40pm.
A Fire and Rescue NSW spokeswoman said the fire was "well alight" and about the size of a tennis court when crews arrived.
Firefighters, wearing breathing apparatus, were on the scene within six minutes.
The fire appeared to be coming from underneath a pier next to Jones Bay Wharf, sending dark smoke across Sydney Harbour and the CBD.
At the height of the blaze, firefighters hit wooden planks on the boardwalk with an axe to get access underneath, where the smoke was coming from.
A firefighting boat from the Port Authority was also sent to the scene, helping to extinguish the blaze from the water.
It is not known what caused the fire, which was quickly brought under control. No one was injured.
Fairfax Media
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