Good weather over summer and an increase in pig-friendly habitat has helped the growth in numbers, while pig hunters may also be illegally letting pigs loose.
Animal Health Board disease control manager Brent Paterson, of Dunedin, said a hunt in the Hyde area in March killed 160 pigs in two days.
"But farmers were telling us within two days there were another 30 to 40 pigs seen," he said.
The hunt was over about 1500ha, and he said the pigs were relatively easy to spot.
There was not an immediate threat of the pigs passing on bovine tuberculosis to livestock but they could pass it on to ferrets or possums, which could then pass it on to cattle, he said.
Pigs are creeping nearer to Dunedin as more land was opened up by farmers and forestry cultivated.
Pigs had been seen at Flagstaff, Whare Flat and behind Brighton.
Department of Conservation technical support officer pests Bruce Kyle said a lot of hill country in Otago had been opened up, supporting more pigs.
Feral pig numbers would, like rabbits, rats, mice, hedgehogs and feral cats, go through periods of growth in the absence of things which normally limited population growth, such as predation, disease, parasites and loss of food or habitat.
Feral pig numbers were kept in check to some extent by pig hunters and were susceptible to bovine tuberculosis, but they coped with it - and parasites - reasonably well, he said.
Probably the most important factor in the population boom was the increased rate of rough hill country conversion over the past 10 or 20 years, Mr Kyle said.
"That has provided the pig, an omnivore, with improved food sources and good cover," he said.
A "patchy mix" of native forest, grassland, pine forest, scrub and improved pasture was pig heaven, and gave pigs a good choice of protein throughout the year, from bracken, briar berries and speargrass root through to huhu grubs, lizards, young rabbits and carrion.
"They're extremely resourceful, so a greater carrying capacity in the landscape via more diversity, more biomass and more soil fertility, potentially means more pigs."
The huge reduction in possum numbers for tuberculosis control in recent years might even have provided pigs with a nice big niche to fill, as possums were also surprisingly omnivorous.
Possums were probably at their lowest level in more than 50 years, thanks to the Animal Health Board's tuberculosis programme, he said.
Experienced pig hunters could do a great job in assisting landowners to rid their properties of pigs, but unfortunately some were being irresponsible by releasing young pigs into new areas, Mr Kyle said.
The Wildlife Animal Control law allowed Doc to prosecute offenders, with a fine of up to $50,000, and Doc would follow up on information provided by the public.
Feral pigs in large numbers were very destructive.
Pig hunter Kim Hiko said it had been a good breeding season and more pigs seemed to be around.
As forestry matured, pigs moved into new areas, so there were more chances to find pigs.
Pigs tended to be bigger but that might be because of the good summer, with plenty of feed.
Federated Farmers Otago president Richard Burdon said numbers seemed to be increasing in places but most farmers welcomed pig hunters on to their property.
He had heard of pigs damaging livestock and digging up paddocks.
Pig hunter Jim Columb said there were a lot more pig hunters around.
The size of pigs had grown this season and they could survive through the winter as they shed fat.
