I have not spent over $40,000 in a year on this card, so have no first-hand experience.
However, the card T&C says: ...
[SUP]9[/SUP] If, during any anniversary year of your Account (which commences either on the date your Account was opened or on the anniversary of the date your Account was opened), you spend at least $40,000 on eligible purchases on your Hilton HHonors Macquarie Platinum Card, you will be upgraded to Hilton HHonors Diamond status. You will retain Diamond status for the anniversary year in which it was earned through to the end of the following anniversary year. Thereafter, in order to maintain Diamond status, you must meet the annual qualifying purchase requirements during an anniversary year with your Hilton HHonors Macquarie Platinum Card or meet the standard requirements outlined in the Hilton HHonors Program at HHonors.com/Terms ..."
I have no idea about how HHonors monitor the "anniversary dates" for credit card opening. However, it wouldn't surprise me if Diamond status would remain until 31 March 2019 (i.e., the usual status cycle), for reaching Diamond from 1st January 2017, unless the HHonors system is sophisticated enough to identify credit card account anniversary dates for each card holder.
Others may have first hand experience.