Response from ANZ:
Thank you for your enquiry regarding Status Points.
Using your example, if you were 5 Status Points short from earning Elite Tier and you earned 400 points from a flight, the 395 will not be counted towards retaining you at Elite. Regrettably you would lose them. However if you find that you are within 5 points short then please do email us and we can have our Tier Status Coordinator review your account and look at your options.
Ouch. I guess one's path through the tiers should include consideration for minimizing SP loss...