The Country Fire Service says a fire burning on the west of the island is burning through a national park, and "firefighters are now unable to prevent the fire spreading".
An emergency warning is currently in place for one fire raging through Flinders Chase National Park, while a second fire on the island that did hit emergency level has now been downgraded.
"You are in danger. Act now. Identify where you will seek shelter, preferably in a solid building. Only leave if your path is clear to a safer place," the CFS said.
"Heat from the fire can kill you well before the flames reach you."
The CFS said the fire was burning near the Ravine des Casoars Wilderness Protection Area.
"Do not leave or enter this area in a vehicle or on foot. It is too late to leave as the roads will not be safe," it said.
Another fire at Duncan further east was downgraded to watch and act on Friday afternoon.
The CFS earlier said the Ravine fire posed "a very real risk to all people and properties on the western end of the island".
Footage of a so-called "fire twister" has revealed the intensity of the flames on Kangaroo Island.
Local resident Brenton posted video to social media of one of several fire twisters breaking out in the inferno, as well as kangaroos fleeing the flames.
The fires have burnt about 14,000 hectares within a 60km perimeter, and spotter aircraft were deployed this morning. About 150 CFS personnel are on the island.
CFS state duty commander Brett Loughlin said firefighters - who have been supported by water bombers - were facing "truly horrific conditions".
"Today is going to be a long and difficult day … the fire on Kangaroo Island is going to be incredibly difficult for our firefighting personnel," he said earlier this morning.
"We're hearing that the containment lines have been challenged this morning. We have observation flights going up right now to ascertain the extent of that.
"If you're to the south and the immediate east of this fire, under these northerly winds that are forecast today you are in an area that we would hold concern over."