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If the old contract doesn't apply anymore, then they don't have any right to own it or transfer ownership anymore.
Ngārimu Blair, deputy chair for Ngāti Whātua Ōrākei people, said most of the artefacts belonging to the tribe were looted by early “treasure hunters”. This, along with urbanisation and displacement, has left the tribe with only a handful of significant artefacts. 
“We have so very few of these taonga and treasures left in our possession,” he said. “When something like this comes up where we’re both excited, but also that sorrowful that we lost so much.” >


Blair cited the example of a carved pounamu (greenstone) club, or 'mere', which is part of the auction. According to a newspaper report, it was originally given by Ngāti Whātua chief Pāora Tūhaere to a British vice-admiral in 1886 on the condition that it remained in his family. Blair says now that the 'mere' has passed out of the family’s hands, it should be returned. 

Intergenerational takeback-sies probably isn't going to happen. Not the best example to focus on if they want people to support their case. 
The looted shit, however - more likely.