Been using (People's dual # sim) China Mobile H.K. - recharge it locally or overseas (U.S.) every 180 days myself, online via their portal - secured processing via Fubon Bank, HKD $50 or more (under USD $7 with transaction/exchange fees already included) or I don't even need to buy the prepaid PINS. Short codes worked here so I can get my balance, expiration date, etc. right on the device - and roam here via both AT&T and T-Mobile (voice & SMS.) As I recall for NWM, if you have contacts in HK - they can also refill it for you via HSBC's ATM terminal, check the vendor's website for details.

(Update - my cc provider billed me USD $6.43 for the last HKD $50 refill charged, a very competitive exchange rate of $7.77 to 1, plus 19 cents in foreign exchange/transaction fee - for extending & rolling over my unused balance for another 180 days)