Hedges are best maintained on a routine basis, but when they have been let go for a few years reduction or reshaping may be required.
What can be done depends upon the type of hedge. Frequently cypress hedges may need a metre or two taken off the the top. This will not impact on the growth of the sides. Other plants frequently used in hedging such as pittosporum tenuifolium should only be cut heavily when absolutely necessary, as wood borer will certainly enter any cut branch, which will lead to dieback of the plant.
We use specialized equipment when performing hedge reductions. A hedgetrimmer cannot cut thicker than about 5mm without damaging the plant. Chainsaws also do a lot of damage on any branch less than about 50mm. We use a special pruning saw that leaves a very clean cut and is a lot faster than any other method.