Often do SYD/MEL to HKG and connect overnight to BKK or DPS on CX. On the last trip check-in desk wanted to collect tax since it was not on the ticket. I pointed out the less than 24 hour rule and a supervisor was called. She said, as Rich indicated, that it was a calendar day not 24 hours and departed with my credit card to collect the tax. She returned a bit later, said she as wrong and that no tax was due in any connecting period of less than 24 hours. Hong Kong airport literature also has the wording "no airport tax due for any stay in HKG of less than 24 hours".