"Even if that is the case these animals are going to suffer and hundreds will likely die."
He said the conditions on board meant many sheep would end up sick or starving to death.
The organisation was also concerned about the animals' eventual slaughter, Mr Kriek said.
Live shipments "cannot be anything but cruel", he said.
Safe would be rallying for support against the shipment this week.
Green Party co-leader James Shaw said the 50,000 sheep revelation highlighted "massive inconsistencies" in the Government's live animal export policy.
The policy was a "farce" that was currently in "tatters", he said.
It was impossible for the Government to verify no sheep were slaughtered on arrival and not all 50,000 would be looked after humanely during the trip, Mr Shaw said.
"The Government is skirting our ban on live animal exports for slaughter by sending sheep and then wilfully ignoring what happens to the sheep when they disembark," he said.
"We need to stop this joke of a trade and foreign policy."
The shipment would be a diplomatic offence to Saudi Arabia following the scandal of 2003, he said.