"All that hard work by hundreds and hundreds of people - because this exploration goes back to the 60s - all of whom dreamed of finding a route from the top of the mountain to the bottom, suddenly we've done it," he said.
"But it's like climbing a mountain peak for the first time. You climb one mountain and go 'yes I'm at the top'. But then you look around and go 'there are other mountains to climb'."
Nettlebed was discovered at the bottom of the mountain in the late 1960s and explored to a height of 800m and a length of 20km.
In 2011, Mr Mckay's group was sheltering from a severe storm when they discovered Stormy Pot, which was 15km long and 700m deep.
When Mr Mckay returned from that expedition he plotted both caves on a computer programme and realised he was looking at the same cave.
"That was an incredible moment. We were on the verge of getting a cave 1200m deep."
Finding a way to bridge the 150m gap between the caves wouldn't be easy. It took four trips over three years before fumes from a kerosene fire lit at Stormy Pot were smelled near a rockfall in Nettlebed, indicating the best place to attempt a link-up.
On a previous expedition, dye tipped into a creek in Stormy Pot had flowed into a creek in Nettlebed, confirming that the two caves were one.
The kerosene-fume expedition last year was funded by Red Bull Media, which made a documentary about it.
When the cavers returned this week, it took two hours of digging to complete the link and enter the cave into record books.
A lot of abseiling is required to pass through a system that varies from "brutally small shafts through to chambers 50m high and two football fields long", and a large river also has to be negotiated.
The cavers spent between seven and 10 days underground on each expedition.
If a new entrance can be found further up Mt Arthur, the cave has the potential to extend to 1500m deep.
But it still won't rival the world's deepest cave - the 2197m deep Krubera Cave in Abkhazia, a breakaway region of Georgia in the Western Caucasus.
