NZIS wouldn't get involved in clearing household effects - it's not within their remit and they don't have the staff or facilities to do it.
MAF will hold stuff so it can be cleaned or if it's not permitted into NZ because it's a bio-hazard.
NZ Customs will hold stuff until the import duty has been paid (if any) or if there are prohibited items or contraband, in which case, the owners will be interviewed and/or arrested.
If this is a true story, my money is on a rogue shipping agent or moving company - we already know that they threaten all sorts in the name of extorting money from the unwary. People, stand up, say no and refuse to pay any more than you have to
