An "extremely large gold system" lies close to current diggings up to one kilometre below the earth's surface, owner Kirkland Lake Gold told investors overnight in an update on exploration drilling works.

Just how many riches are down there is yet to be confirmed but the drills have shed new light on the size of subterranean gold-bearing rock, the mine's principal geologist Simon Hitchman said.

"'Potential' is the key word here," he said.

The mine's engineers are about to start work estimating how many new ounces of gold could be added to the mine's "reserves", Mr Hitchman said.

Kirkland Lake has 15 exploration drills running at Fostervile. Nine of them are underground, Mr Hitchman said.

Drilling has uncovered new information on gold mineralisation at two other underground sites known as the Swan and Cygnet systems.

More: Gold mine continues to ramp up production

Fosterville is already Victoria's largest gold producer and the drilling in the so-called Lower Phoenix System discoveries have taken place in the same year it produced its two-millionth ounce.

Kirkland Lake Gold credited its most profitable three months ever last quarter thanks to the Fosterville mine and is hoping to dig up between 570,000 and 610,000 ounces of gold this year.