Usually low pressure shower heads have very small spray holes and a small diameter shaft (coned) to allow for higher flow rates. Felton (based in Auckland) specialize in low pressure shower systems, they can advise what shower head could be suitable for your situation.

Hope this helps.

Plumber   

Hi, if its a shower handset with hose you can measure using an std bic ballpoint pen, if the hose is for all pressures application the pen will inside it all the way to the clip and the same most times applies to the handpiece as well, a local plumbing merchant should be able to suggest suitable replacements.
Hope this helps