And the fun continues, spend $851AUD for the stay, received 3901 base points indicating a $390USD spend. Current exchange rates have it at a spend of $650USD. I understand you don't get points on taxes, which the hotel appears to be avoiding given my tax invoice says GST was $3.27
Emailed a query, got the following rubbish:
Please be advised that Base Points are the standard points an Hilton Honors member earns for their hotel stay. Members receive 10 Base Points (5 Base Points at Home 2 Suites) per USD spent on the hotel bill before taxes.
In addition, points awarded for stays at hotels within the Hilton portfolio outside the U.S. will be based on local currency converted to U.S. dollars. This rate will be determined at our sole discretion, based on standard currency conversion methods but may vary from currency conversion rates used by credit card companies.
Please note that continued modifications of your stay posting after the departure date can be affected by the exchange rate that is constantly changing. I sincerely apologize as we are not able to provide further modifications to the stay to apply a previous exchange rate.
So apparently the missing $250usd of spend is blamed on currency exchange, which has perhaps varied by 1% at most.