Dr. Harrison followed a massive budgerigar flock on New Year's Eve and said the 'noise was incredible'.
After coming through a tough phase marked by drought and bushfires, some inland Australian bird species are now thriving after a plentiful amount of rain.
Birdlife Australia's NSW woodland bird program manager Mick Roderick said it was hugely encouraging.
"A remarkable boon, it's just been incredible," he said.
"The rains really started in February  and birds that would normally have bred by then — but hadn't because of the hot, dry summer with the bushfires — then started breeding from March, April right through to May and June.
"It really is a case of 'just add water'.
"We've had some really tough seasons for our breeding, endemic birds because of the dry. And now, with the very welcome rains, the birds have really gone ballistic and bred up.
"It really is a testament to how Australian birds have evolved to see out those tough times. And then once the good times come they absolutely take advantage."
Budgies 'breed like crazy'
Mr. Roderick said one species that was noticeably increasing in numbers was the budgerigar.
"In 2019 there were virtually no budgerigars recorded in the state. And in 2020, since the rain came, there were thousands of budgerigars reported right across the inland parts of NSW," he said.
"The birds have been breeding for many months and we know that birds are having second clutches. It's really exciting.
"Budgerigars are one of those quintessential birds that will breed like crazy after lots of rain and we are certainly seeing that at the moment."
'A roar of wing beats' in a flock of thousands
On New Year's Eve, 2020, birdwatcher and photographer Dr. John Harrison reported on Birdline NSW — a site for reporting notable bird sightings — seeing a flock of thousands of budgerigars over Nombinnie Nature Reserve, near Mount Hope in central NSW.
"Counting part of one photo and then extrapolating, I believe that well over 5,000 birds were present," he said.
"I went and sat quietly in the field and they got closer and closer over time. They were feeding on something in the grass.
"Eventually the whole lot flew over my head after a brown falcon came in like a missile hoping for lunch. The noise was incredible.
"The sound of the flock flying was like waves crashing on a distant beach. But up close it was a roar of wing beats, the sensation of turbulent wind, and a racket of chirps all at the same time."
Coastal sightings might rise
Mr. Roderick said if the weather enters a dry cycle again, some budgerigars could travel to coastal areas in search of food.
"They prefer to be in inland Australia," he said.
"But I'm assuming over time the inland rain will stop, conditions will start to gradually dry out, and the food sources won't be reliable.
"When that happens, budgerigars will be forced to move towards coastal areas.
"We saw that in 2013 when we had thousands of birds [budgerigars] along the east coast. That could happen again in a little while, once life gets hard inland again."
'Just so many' as elusive buttonquails increase
Mr. Roderick said another species booming at the moment was the buttonquail.
"We are currently seeing an eruption of buttonquails. Buttonquails are essentially grassland birds, and there were very few records [of them] over the past few years when conditions have been pretty poor," he said.
"But since the rain, we have seen many, many buttonquails.
"Red-chested and little buttonquails, out west, and painted buttonquails on the coast that occur in the forests.
"All three of these species are breeding really well."
Mr. Roderick said it was encouraging news for birdwatchers.
"Normally we wouldn't expect to see red-chested or little buttonquails near the coast. Maybe the odd bird, but not in the numbers we are seeing now, and we are seeing buttonquails breeding in a number of sites," he said.
"It's very exciting for birdwatchers because buttonquails are a very difficult bird to see normally, but right now is a great time because there are just so many of them.
"Australia really does see boom-bust cycles [for wildlife] and we are really in a boom at the moment."