It could mean the new owners will find themselves with no-one prepared to work underground.
The Engineering, Printing and Manufacturing Union (EPMU) delivered the bombshell on Monday to receivers PricewaterhouseCoopers. They were given time for the news to sink in, and will meet them again next week.
The receivers have been actively trying to sell Pike - which has been idle since a series of underground explosions last November killed 29 men - both in New Zealand and internationally.
EPMU national secretary Andrew Little confirmed yesterday that if the mine was sold and the receivers, or the buyer, did not have a viable recovery plan to get the 29 men out, "coal miners internationally will react, and are most likely to say Pike River is not a mine we will work on".
Potential buyers needed to know that the issue of having dead men still trapped inside the pit was a "very real one" for mine workers. The union has already started spreading the word.
The head of the International Mine Workers' Federation was due in Greymouth next month and was taking a close interest in developments, Mr Little said.
When Mr Little was last on the West Coast, in April, the word from union members was they did not want to go back into the mine while the bodies were still there.
It is unclear if a mine has been blacklisted internationally before. However, the mining community internationally has tens of thousands of members, many through the International Federation of Chemical, Energy, Mine and General Workers' Unions.
"The receivers got a pretty sharp message," Mr Little said.
The union supported the New Zealand Mines Rescue approach to re-entering the mine, which could take time to effect a recovery.
Receiver John Fisk, of PricewaterhouseCoopers, said yesterday that at the meeting with the victims' families in Christchurch last week, everyone agreed the receivers would use "commercially reasonable endeavours" to get a commitment from potential buyers that the recovery would not be impeded by the sale.
"It was on our radar already, and it was an agreed approach [on Monday]."
Asked if making the body recovery a condition of sale could impact on the price the mine fetched, Mr Fisk said potential buyers would make their own assessment.
Families' representative Bernie Monk said they were generally supportive.
"It's been a long time coming."
He stressed they did not want industry on the West Coast held up as a result.
"The families are still worried about the West Coast. It's important we get on with the royal commission [of inquiry], and that contractors get paid and people get back to work."
Mr Little said the mining community was tight-knit. Miners closely followed the other recent underground tragedies in West Virginia and Chile.
The level of support and interest in the wake of Pike River remained high, six months later.
"It's phenomenal. We are still getting queries."
