We spent a long hard fortnight in the crater of Mount Bosavi filming the last part of Lost Land of the Volcano. There were a dozen of us - a mixture of biologists, programmemakers and trackers from the Kasua tribe thrown together under a tarpaulin, eating tinned fish and rice and hunched together in damp hammocks.

We had no idea what we would find - if anything. The only plan I had was that day and night we'd search for animals and just film everything that happens. All we had to rely on was a gut instinct that this was going to be a good place. The crater has walls half a mile high and even the local tribespeople from outside the extinct volcano said they wouldn't come here. "Why bother? They said - it's far too steep."

If you want to imagine how it looked - think of the Swiss Alps covered in dense jungle. I'd look up and see a stream plummeting down the mountainside almost a vertical kilometre above me.

The very first day we had a great sign. A Doria's Tree Kangaroo wandered a few metres past camp. New Guinea is an Alice Through the Looking Glass rainforest – none of the creatures are as you expect - and these are among the oddest: a kangaroo that has evolved to live on the leaves in the trees, it looks like a big teddy bear with a very thick tail. Boy, are they normally hard to film!

Wildlife cameraman can spend weeks or more trying to catch one in the wild – and still come back with rubbish rushes. We grabbed cameras and chased after it. Chase is the wrong word. The slopes are almost vertical and thick in mud. But then, after half a mile of deep panting she was found sitting in a tree. We wedged cameraman Gordon Buchanan in a tree opposite, with me in the one next to him so I could film him in action, and then we let the cameras roll.

From then on it got mad. As biologists and filmmakers, we are used to disappointment. In most places animals are thin on the ground and leg it as soon they sniff humans. But here, the animals were in high densities and just did not seem bothered by us.

We found the cuscus a few days afterwards. Cuscuses are a strange group of animals cursed by sounding like an item on a Moroccan restaurant menu. In truth they're a group of shy and sweet marsupials that generally live high in the trees feeding on leaves. Looking a bit like a teddy bear, ecologically they're part monkey, part sloth. Dr Kris Helgen and Muse Opiang were the biologists with us – and as soon as we had this one in camp, clambering all over us, they knew it was a new type.

To be holding an animal totally new to science – that's one of the special moments in my life. If you work in the tropics you are bound to find new species of insect or spider (somewhere there is a fly with my name on it). Even a new tiny mouse or micro bat is not too uncommon. But a big mammal – and so cute as well! Later on it turned out to be a new subspecies of the silky cuscus.

Then the team found the giant rat. I had never seen anything like it in my life – at first glance more like a beaver than a rat. And again, it sat quietly in camp, chewing on a fern and wondering what all the fuss was about as we rushed around him filming and taking photographs. Mammals with no fear of humans. In the 21st century that is a rare sight indeed. Now the talk is how to promote and conserve this remarkable crater – perhaps by making it a World Heritage Site.

• Steve Greenwood is the series producer of Lost Land of the Volcano, which will be shown tonight on BBC1 at 9PM