For the Nth time! EVERYONE who has a salary in Hong Kong is subject to tax if they earn more than the tax free allowance. In pretty much every country tax is calculated based on your income in a 12 month period (a tax year). In some countries the tax year is the same as the calendar year (January to December) but in others it starts at a different point. In Hong Kong the tax year is April to March, so your tax is assessed on your income in the 12 month period between 1 April one year and 31 March the next.
In the tax year 2007-2008 you will be here for 5 months (Nov 2007 to Mar 2008) so your earnings will be $125,000. The tax due on that is HK$500.
In the tax year 2008-2009 you will earn 7*25k = HK$175,000. Assuming no change in the tax rates or allowances then you will be liable for tax of HK$3750.
So in total you will have to pay HK$4250. (You get lucky because you work year is split over two tax years - if you started on 1st April you would have to pay HK$23,500 in tax.)