There’s a little lake I’ve known for 25 years now. In many ways it’s unremarkable, but what makes it unique to me is how much time I’ve spent there: not only is it 5 minutes from my home, it’s also where I guide and teach.

Although it looks like a natural feature, the lake is manmade and in human years, it’s old but not ancient. I can find it on an Army Survey map from 1975, and it may be older still. It was once a swamp under an ancient volcano, lying amidst a long-gone forest of giant trees. According to a swagman’s diary I read, this forest was apparently a fearsome place where Gold Rush bushrangers lurked.

The giant trees are limited to small clusters now. Most were felled in the early 1800s to make way for crops and livestock that still thrive thanks to the rich, deep soil and generous rainfall. Well, mostly generous. Even this gentle landscape is not immune from drought, and the motive to originally build this lake was not fishing, but backup water for crops. A local farmer tells me that in the late 1970s, potatoes irrigated from this lake could be sold for $700 a ton. “We’d harvest a row,” he says, pointing wistfully to a big paddock on a flat above the lake, “And we’d have the value of a new Holden in the trailer.”

Today the paddock is for grazing Angus, and potatoes are worth a fraction of what they were in those boom years. Still, knowing the history helps understand how a 10 acre lake could be built back then when no one could justify constructing it today – not for crop water anyway!

For the past quarter century, the lake has been all about trout – a private water, dedicated to flyfishing. It’s been gradually modified from a ‘spud dam’, to something that looks so natural and ideal for fishing, it’s hard to believe it hasn’t been there forever. As I write, it’s brim full, pushed back against the surrounding rocks and rushes like a Rubenesque model in a pair of size-too-small jeans.

I just realised that analogy is not accidental; the lake in this state is fertile and bountiful. Not only do the trout grow fat, but the midges buzz and the frogs croak so deafeningly, on evening you wish you could turn the volume down a little. In seasons like this, it’s easy to forget that, even without the demands of agriculture, the lake became so low during the Millenium Drought, you could have driven a car over half of it without getting the tyres muddy. Fortunately, the lake in its current state appears to have flooded a much older one, so there’s been enough water and depth in the worst droughts (so far) for the trout to survive.

Visitors often comment that the lake must fish better when full than low – new arrivals rub their hands appreciatively when they see the water level as it is now. Conversely, when it is low and a lot of the lakebed is cracked mud or grass, they can’t help hiding their disappointment. However, the strange thing is, the lake can fish just as well when it’s shrunk to less than half its present size. “Aha!” some anglers exclaim when that’s pointed out, “Less water between the fish!”

Of course, it’s not that simple – if it were, the fishing would be twice as difficult when the lake is full. Like many things in life, the trout and their behaviour don’t neatly conform to a set of statistics. I’ve seen the lake shrunk back to a few acres of water, yet the whole surface boiled with midging trout. Then, in its present state, the fish can crowd the shallows; one every few metres. It’s even more remarkable when the lake first floods the long-exposed grassy flats on the eastern and southern shores. For the initial few hours after inundation, normally wary browns of 5 pounds and more, invisible for much of the year, can be found rolling and frolicking like excited puppies, the bounty of flushed worms and grubs drawing them into water barely deep enough to cover their golden backs.

And conversely, the lake at any level can appear devoid of trout. Once, a few years ago, after a fortnight of not seeing or catching a single fish, my fellow guides and I seriously began to wonder if some invisible plague had decimated them. Finally, we were relieved to find the trout alive and well when a huge cloud of daphnia – maybe an acre across – drifted close to shore. What we had thought was dirty water, was in fact a bloom of billions of these tiny zooplankton. Once it was close enough to observe, we could see dozens of big browns and rainbows gliding gracefully though the cloud, gorging like baleen whales.

Beyond the fishing, it’s not difficult to observe the other wildlife that thrive in and around the lake. When you’re quietly waiting for the fish to start rising, or sitting on the lakeside cabin veranda having a break, you can see there’s a whole other world of life besides the trout. A pair of black swans nest on the lake every year. Once, at the end of the drought, they built their nest – a great mound of rushes – too far out in the lake. When the rains returned, the swans tried frantically to keep their eggs warm and dry as the water rose, but then finally swam off, defeated, as a stormy day pushed the water level a fraction too high. For days afterwards, six dead eggs bobbed around the lake; a reminder that even waterbirds don’t always get it right.

But then, to our amazement, the swans built another nest a few weeks later in a more sensible location and successfully hatched half a dozen balls of grey fluff no bigger than my fist. The swans are committed parents and will fight off foxes and hawks, so in a matter of months, all the youngsters had grown to be almost as big as the adults. And then one late spring day, the parents turned on their brood as they always do eventually, chasing and pecking at the bewildered young swans until, after a few days, the juveniles gave in and flew off to find their own lake.

There have been so many other sights that stay with me: watching a peregrine falcon teaching its offspring how to hunt; the canny swamp harrier attacks on moorhens and ducklings, using island forests as cover for a last second surprise. Grunting koalas and brawling possums as we walk back from the evening rise along darkening paths.

Most of the fish memories blur over time and I must consult my diary for hard facts. But a couple of memories stand out and perhaps one most of all. I was guiding and drifting rather aimlessly in a rowboat on a bright, still November day. Not much was happening. Perhaps we’d caught a couple but I can’t honestly recall. I do remember we’d reached that point in the session when the lack of activity and a warm sun had me thinking about a lunch break. I absently noticed a mayfly dun sitting on the oily water about a rod length from the boat. Then, the biggest trout I’ve ever seen in the lake nonchalantly cruised up from the depths – a great, dark shadow – snipped the dun off the top and sunk back down out of sight.

There was a moment of incredulity before my guest and I swung into action, me frantically changing his wet fly for a Shaving Brush, he then whacking the fly on the water ahead of the direction in which the trout rose. Two or three more casts followed before we came to accept that the huge brown was a ‘oncer’.

And here’s the thing. No one had caught a trout remotely as big in the preceding seasons, and nothing remotely as big has been caught since. I thought I knew the lake well, but a trout as long as my arm had lived there (maybe still does?) and I only ever glimpsed it once.