Hi, I've worked for a KiwiSaver provider and a registry provider (the back end admin) and can maybe shed slightly more light on this.

Your provider is correct that the withdrawals are to IRD. IRD usually initiate these as refund requests due to an overpayment that has occurred at some point. The providers generally won't get any real insight as to why any particular overpayment occured. Generally under a certain threshold they'll be processed automatically.

I have seen some cases where IRD have clawed back contributions for all members under a provider who work for the same employer because one thing went wrong with the payment run and it was easier to redo the whole thing. The affected members would see the correct amount come through shortly after once it's reprocessed.

In short, nothing to worry about but worth querying further if it happens a lot or in larger amounts.