The peak came in March this year. Since then the price has dropped 50 per cent, to the point where a tonne of premium logs is worth $60 less.
China uses 1 million tonnes of logs every month. There were 4 million tonnes sitting on Chinese wharves at the end of April.
"They drop the price to supply the logs because that's the only way to reduce the volumes," Mr McCarthy said.
The drop has had a dramatic effect on New Zealand's timber industry. It has made the harvest of some trees uneconomic.
Nationwide more than 100 logging crews (generally of about eight people) are out of work. Competenz, the industry training body for forestry, has had 354 training agreements cancelled.
In Wanganui 15 crews have been laid off. Most of them were harvesting smaller farm woodlots. Those are usually the first to cease production when prices drop.
Bigger corporate forests often have large long-term contracts and manage to trade through bad years. Some, such as Arbor Forestry and Ernslaw One, own sawmills and produce for the domestic market. They are less affected.
People who don't have to harvest at times of low return can leave their trees to grow until prices increase.
"That's the benefit of growing pine trees as opposed to growing wheat or something that only has a certain lifetime."
Pinus radiata can be harvested when trees are anywhere between 27 and 40 years old.
The logging crews harvesting smaller woodlots on farms tend to be newer and less experienced. Sometimes they're less careful about health and safety.
"Some of the woodlot operators are very good on health and safety and some of them are shocking," Mr McCarthy said.
When newer crews are laid off their workers' training is interupted and they stop getting experience. This can create a vicious cycle, because when work resumes the inexperienced workers are back doing the dangerous work of felling and harvesting trees.
The fact some of them are not at work may be the reason New Zealand has had fewer accidents in recent months.
The owners of corporate forests audit the crews working there to make sure their procedures are up to scratch.
"A good and polished contractor works his way into the corporate forests."
Prices are sure to bounce back up again, perhaps in six months. In the meantime Mangoihe Logging is in survival mode but managing to keep staff in work, because it mainly works in corporate forests.
"It's a downturn over which we have no control. If timber isn't harvested we can't shift it. But it's quite nice to finish at lunchtime on a Friday every now and again," Mr McCarthy said.
It's tougher on others. Mangoihe has had 37 unemployed forestry workers asking for jobs in the past month, and some of them were very experienced.
McCarthy Transport, co-owned with Steve McDougall, has still got all its 82 logging trucks hauling timber to ports and mills too. That's because they are mainly working for corporate forests or for forest owners that own sawmills and process timber here.