A Perth-based company has struck paydirt in Southern Africa, finding a diamond worth more than $20 million.

Lucapa Diamond Company found the massive 404-carat diamond at its Lulo Diamond Project in Angola's Lunda Norte province, in the country's north-east.

It is the biggest diamond ever found in Angola.

Lucapa chairman Miles Kennedy said it was a huge achievement.

"When we first looked at the property, 3,000 square kilometres of untouched ground, 700 kilometres inland from the coast, you are talking about a very, very remote area," he said.

"The results today are a wonderful vindication of eight years of pretty hard work.

"It's the biggest diamond ever recorded from the country of Angola and it's the biggest diamond ever recovered by an Australian diamond miner.

"So, we have hit a number of firsts finding this diamond."

He said the "spectacular" white diamond, which was considered to be of excellent quality, could be valued at more than $20 million.

"We're not used to valuing 400-carat diamonds, but if we look at other diamonds slightly less weight than this, you're looking in the order of $20 million," Mr Kennedy said.

Mr Kennedy has a long history of searching for diamonds, having set up the Ellendale diamond mine in Western Australia's Kimberley in the early 1990s.

He took his geology team from the Ellendale mine to Angola when he set up Lucapa.

"I started the Ellendale diamond mine and was there from 1993 to 2007, so 14 years, and in that time we only got one diamond greater than 10.8 carats, known as a special diamond," Mr Kennedy said.

"With this company in Angola, we have now found over 100 of these special diamonds in the last six months."

He said the huge cash injection from the diamond sale would allow the company to expand its operations in the area.