These pages are relevant:
This one lists persons who are exempt from MPF:
http://www.mpfa.org.hk/english/abt_m...ms_cov_ep.html Expats with pension schemes elsewhere are covered by the fifth point ("Persons from overseas who ... are covered by overseas retirement schemes").
But this document describes the application of the MPF Ordinance:
http://www.mpfa.org.hk/english/leg_r...files/IV15.pdf
It states that the only people who can be exempted are those who reside in HK on an Employment Visa AND stay for less than 13 months or are members of a similar overseas scheme.
The document says explicitly that anyone residing in HK on any other basis MUST pay MPF. That includes all Permanent Residents and, interestingly for some here perhaps, all Dependant Visa holders who are working (regardless of whether they have an overseas pension scheme).