Mice can swim
A big carpet snake, say 12' will eat 4 mice once a week.
IT was forseeable
West of the hill has been in drought for 4 to 11 years depending upon the actual place.
Thus nearly all of the natural preditors were in short supply.
Then we had the bushfires which killed around 1/3 of all of the birds so no birds that feed on mice
Then there was flood so lush vegitation & the first species to recover are the imported mice , the native species breed a lot slower .
Usually with a flood / drought cycle it would take 2 to3 seasons for mouse numbers to stabilize
However the fires cooked a massive number of snakes , dingoes and other small carnivores as well as burning down the nesting sites for owls, hawkes, eagles & other birds that eat mice.
Then there is the problem of what to do with the dead mice.
Mark ends up with a pile around 10' wide 8' high & 20 ' long most nights
These have to be burned or they become breeding places for blow flies.
Excessive blow flies lay eggs in the dags hanging off the sheeps bums and the maggots then crawl into the sheep and eat it alive from the inside out.
Not a pretty sight and can knock your breeding flock down to nothing in no time flat.
They do the same to the vulvas of ewes after they give birth so you loose the mother and of cause the lambs then starve because mum is dead