Most homes that have poor shower pressure have a water supply tank mounted, either on the hot water tank itself, or in the roof space, or they have a pressure reducing valve supplying water to the hot water cylinder. In rare cases the hot water pipe work is too small. There are several methods of increasing the hot water pressure at the shower.

1. Change the shower mixer $350 to 500 $ approx, depending if the wall lining has to come off, most times it does not.
2. Change the shower rose. $25 to 100 approx
   

The method chosen is dependent on how much water you want at the shower, how much it costs to do that, and what is possible. To find out which is best requires a plumber look at the situation, and list the best options depending on your budget.