The diamond came from the Lulo mine, 40 percent owned by Lucapa.
Lucapa shares rose 29 percent on Monday after the announcement.
A photograph released by the company showed the stone measuring about seven centimeters in length.
Shares in Lucapa, which is based in West Perth, were 9.5¢ higher at 41.5¢ in afternoon trading on Monday. They have held the position in Tuesday morning’s trading.
Lucapa chief executive Stephen Wetherall said the company would continue efforts to find the source of the huge diamond.
The find came just months after another company, Lucara Diamond, found a 1111-carat diamond in Botswana.
It was said to be the world’s second-largest diamond, behind the 3106-carat Cullinan diamond found in South Africa in 1905.