Both sites were contaminated by the gold extraction process. When the quartz rock was roasted to extract the gold, arsenic occurring naturally in the rock was released as a gas. Early photographs of the mine site show dead trees surrounding it.
Davies said about half the arsenic produced at Prohibition had ended up as a herbicide on paddocks in Christchurch.
"They made so much arsenic as a by-product that they made far more selling arsenic ... for horticultural purposes," Davies said.
Cleaning up the sites - which are about 5km apart - had cost $3.6 million, jointly funded by DOC and the Ministry for the Environment.
Twenty workers spent 12 weeks on each site. They had to wear protective clothing and required an external air supply.
All vehicles had to be washed before leaving the sites.
The most contaminated material was packed in 200 litre barrels designed for nuclear waste, trucked to Christchurch for processing then dumped in an approved landfill.
The rest was buried in lined containment cells on the mine sites.
Davies said the Prohibition clean-up finished last winter.
Cleaning up the more isolated Alexander site finished in February.
Both sites were now open to the public.
He expected Prohibition would attract more than 10,000 visitors a year, increasing to about 30,000 a year over the next five years.
Alexander, which was a 40-minute walk from a road end, was likely to attract hundreds of people a year.
Prohibition operated from 1934 to 1958. Its pit, described as "New Zealand's biggest hole" descended 829m.
Alexander operated from 1924 to 1943.
Environment Minister Nick Smith has previously described the "acutely toxic" mines as a "blight on New Zealand's clean, green reputation".
"The levels of arsenic are among the highest recorded anywhere in the world at 400,000 parts per million on land, or 500 times the safe level, and in water at 300 parts per million, or 33,000 times the safe limit for drinking water," he said, announcing the clean-up in 2015.
Symptoms of poisoning
• The symptoms of arsenic poisoning include a metallic taste in the mouth, excess saliva, problems swallowing, blood in the urine, muscle cramps, stomach cramps, hair loss and convulsions.
• Treatment focuses on using drugs that bind the arsenic. Both are then excreted through urine.
• Acute arsenic poisoning can kill. Those who survive will usually have nerve damage, and many suffer cardiac, liver, renal and skin problems. The prognosis is fair to poor.
• Chronic poisoning and organic arsenic exposure have better outcomes with fewer and less severe problems.
- Westport News
