It was the right decision. There had been a lack of judgement by getting involved with applications from family members. And while she did not ask anyone to bend the rules for her, they did, and that reflects adversely on the culture of the NZ Immigration Service which she headed up. And she put staff in a difficult position by even getting involved.
By resigning, Mary-Anne is putting the reputation of the agency and public service, ahead of her own security. She could have demanded a negotiated settlement but chose to simply resign.
As I have mentioned before she has had an excellent career in the public service, and it a shame to see it end over uncharacteristic errors of judgement. I hope she does very well with her future endeavours.