It has been described as the "first glimpse" of a new world-class gold system beneath Kalgoorlie's famous Super Pit.
Despite more than a century of mining on the historic Golden Mile, there has been limited exploration outside of the rich deposits which have yielded more than 21 million ounces since the Super Pit began production in 1989.
Underground access portals on the western wall were developed last year to provide new drilling platforms for testing north-west of the existing pit.
The work represented the first significant underground mining activity on the Golden Mile in more than 30 years.
The investment by Perth-based gold miner Northern Star Resources is starting to pay off after it said it hit pay dirt.
In the company's annual reserves and resources statement to the ASX this week, Northern Star said drilling had increased the underground resources 20 per cent to 5 million ounces.
Northern Star managing director Stuart Tonkin said the portals had been developed 1.5km in length and drilling only began last November.
"It's really not been a lot of time drilling and we're obviously continuing to drill now," he said.
"But we've already added a million ounces of inferred material."
He said there was 5 million ounces of Fimiston underground resource.
"So again it shows us the thin end of the wedge," he said.
Deepest workings 1.4km underground
More than 3,500km of tunnels and shafts have been created underneath the Super Pit — equivalent to driving from Perth to Sydney- in a century of mining the area.
The deepest historical workings extend about 1400m below the surface.
But drilling has hit gold mineralisation as deep as 2km below the surface.
Northern Star has made no secret of its ambitions to eventually restart underground mining and is taking delivery of 39 new haul trucks as part of a $250 million fleet overhaul.
"We're really only looking at the one quadrant at the moment," Mr Tonkin said.
He said the company would put more drill drives to the south as well as onto the eastern side beneath the plant.
Mine life beyond 2035
The current reserves at the Super Pit and neighbouring Mt Charlotte mine stand at 11.9 million ounces.
There are 27 million ounces of resources which require further geological work to prove up as reserves under the JORC Code.
The significance of the latest underground drilling results is the fact they are outside the current mine plan, which will result in further cutbacks of the Super Pit until at least 2035.
"This is currently not in the plan, so it's important we get in and do this work over the next couple of years to define it and start to work out the scale and magnitude and how it can come into the mine plan," Mr Tonkin said.
He said he was not surprised by the latest find.
"We're really confident we have multiple decades ahead of us," Mr Tonkin said.
"There is no geological reason why this terminates at depth."
The drilling results were released as Northern Star prepared to announce the results of a feasibility study into a potential expansion of the Fimiston Mill in coming months.
The Fimiston processing plant is one of the biggest in Australia and was commissioned in 1989.
It has since undergone two expansions and treats more than 13 million tonnes of ore a year from the Super Pit and Mt Charlotte.
Mr Tonkin said expanding to 23 million tonnes a year was among the options being considered.
