From the Wild Dog Mountains to the Wolgan Valley, Blue Mountains bushwalkers have witnessed a major mouse problem.
The tiny, but scary, Mus Musculus Domesticus - also known as the house mouse - about 13 grams on average, has made its way to previously unseen wilderness locations on the outskirts of the Blue Mountains.
Several walkers have told the Gazette they have reported seeing dozens of mice near their food and tents when packing up after a night in these locations. Hikers have reported hungry mice even chewing through their tents and backpacks to get to food. Campers have taken to hanging their food bags in trees.
CSIRO Health and Biosecurity senior research officer and resident mouse plague expert, Steve Henry, has been tasked with dealing with the plague. He had not heard mice had spread to wilderness in this region but was not surprised.
"Well they [the mice] are a long way from grain, not a long way from food. They are everywhere where humans are. I've heard there are mice in Antarctica in the base station, we take them everywhere we go, they are wonderful stowaways."
Canberra-based Mr Henry is touring northern NSW for three weeks, helping farmers with the plague. He said mice in the Mountains would be surviving on a "heap of seed in grasses and shrubs, they will all be seeding, they will be dining out on all those, or invertebrates [small insects, beetles and bugs].
"They are highly adaptable critters and in fact in New Zealand, mouse outbreaks are closely tied to a species of tree that has a major seeding event."
He said small insects were "protein packs" for the mice. Mice numbers are in plague proportions through the northern NSW cropping zone, into southern Queensland and are now being seen through central and southern NSW.
"It's not cause they are moving, it's because conditions have become favourable to mice and they are breeding up to high numbers."
Farmers can remove 400 mice from one house in one evening, he said. A mild wet summer had made the breeding extra successful.
"Conditions have been favourable - it's been a pretty mild summer ... good rainfall events and that has kept them breeding right through into autumn."
He said while mice might only live a year, a single pair can give rise to 500 mice in a season. He is worried about the high level of juvenile survival - mice breed from six weeks.
"We're hopeful it might happen soon [the end of the plague], we just don't know, when mouse numbers are very high they are interacting and that facilitates the spread of disease - when they run out of food, they turn on sick and weak ones and eat each other and eat the babies and that causes the crash."
Mr Henry's focus has been the cropping sector and minimising impact to farming systems and the winter crop.
He is giving a round of speaking engagements throughout NSW, advising farmers to reduce residual food, and put out bait as the winter crop is sewn.
"It's pretty nasty stuff, it has significant psychological effect [on farmers]."
He said in the Blue Mountains "winter will slow the mice down, so they will stop breeding and some will die with less food, but what we are concerned about is the level of winter survival, if conditions are favourable and they breed from a higher population base".
Mr Henry said "historically mice haven't been a problem in those environments that I know of. I suspect they will disappear in time, but we haven't studied it."
Blue Mountains Climbing School' s Hugh Ward said he had heard reports and seen the mice problem for himself. He and his wife Bridie Campbell "did indeed see a number of mice on our walk from Kanangra to Katoomba a couple of weeks back", he said.
"There were lots around Mobs Soak, Medlow Gap, and on the ridges leading up to Tarros Ladder."
"[Fellow rockclimber] Adam Darragh got some vids of mice out at the end of the Narrow Neck during the UTA [Ultra-Trail Australia] rigging he was working on recently which were quite cool," he added.
And Barker College's joint director (facilities) at The Grange in Mt Victoria, Liz Charlton, said she had been camping for two decades in the Blue Mountains wilderness and had not seen anything like it.
"We were recently out at Mobbs Soak in the Wild Dog mountains and the mice were everywhere. Luckily we had the heads up from another local bushwalker and the kids were able to hang all their food up on thin cord tied between the trees. We had mice running under and over the tents and everywhere in between."
"I've been hiking and camping in wilderness areas of the Blue Mountains with students for over 20 years, I've never seen mice in these areas. Last week in The Wolgan Valley, kids were yelling and screaming out in the night from their tents, the mice had got in and were eating things like toothpaste and a packet of panadols in one student tent. We are hopeful the cold snap this week slows the mice up."
Mr Henry has advised bushwalkers to "keep your food in hard plastic containers and your tent zipped up".
He also said anyone with a car should "make sure you close the doors of your vehicles all the time. They eat wiring out of the cars".
Some work has recently been done on mice in the wilderness of New Zealand, which they would consider in relation to the problem, Mr Henry said.
The CSIRO's work is funded by the Grains Research Development Corporation.