About 50,000 sheep - New Zealand's largest live sheep export shipment for nearly a decade - are about to leave Timaru for Mexico.
The Ministry for Primary Industries (MPI) has approved the export of the sheep, as well as about 3000 cattle, for breeding purposes, due to high demand in Mexico after a recent drought.
Since 2007, livestock cannot be exported for slaughter unless special approval is granted by the Director-General.
PrimePort Timaru chief executive Phil Melhopt said the livestock-carrying ship NADA should be loading already and would set out on its two-week voyage early tomorrow.
SAFE executive director Hans Kriek said the journey would be stressful and could cause the animals to suffer.
Mr Kriek said many of the sheep were unlikely to take up pellet food and would starve.
He said, since 1981, over 2.5 million livestock have died in export ships from Australia.
MPI deputy director-general Scott Gallacher said vet staff would be checking the stock before they left.
"If any of our staff have any concerns around the fitness of the animals, the health and wellbeing of the animals, we're very clear in terms of the steps that we'll be taking...
"The wellbeing of those animals is the most important consideration for all of our staff at all times."
Mr Gallacher said the exporter - Mexican-owned, Christchurch-based Livestock and Agricultural Products - had signed a statutory declaration that the animals are not intended for slaughter.
Prime Minister John Key said the expect was required after tough conditions in Mexico - and it was not uncommon for New Zealand to export large numbers of livestock.
"It's a combination of, they need big numbers, because they lost so many in their drought, and because that ship can accommodate that. That was what they wanted," he said.
"It's a large number but it's not the first time we've exported reasonably large numbers. It's at the bigger end, that's all."
He said hundreds of thousands of livestock were exported for breeding, with many of those going to China.
Livestock and Agricultural Products has refused to comment about its shipment.