Prune Hard in the Spring for stem and foliage effect.
Spring Pruning will rejuvenate the shrub to provide stronger stems and shoots which in turn will produce better foliage throughout the summer months to come.
Eucalyptus gunni, Catalpa Aurea, Cotinus, Dogwoods, Sambucus, Robinia frisia, etc. will all benefit from a spring pruning.
Hard pruning in the Spring for Shrubs that flower late mid-late summer:
Tamarix pentandra (T. ramosissima)
Not Tamarix tentandra which requires different treatment, Hydrangea Paniculata types.
Also Buddlejas, Hardy Fuchsias, Hypericum, Lavatera, Prunus triloba (right after flowering), etc.

To rejuvenate old shrubs
or bring back into line shrubs such as Brachyglottis and Hypericum. Most shrubs can be given a new lease of life by cutting back hard. However, realise that the shrub will not stay small simply because you have given it a short back and sides. It will spring back into growth with a new enthusiasm.
Don't forget that it now has a root system that was used to supporting and feeding the old large shrub. It will 'pump' soil moisture and nutrients to ensure that the shrub gets back to 'normal size' as soon as possible.
Cutting shrubs back hard to near the ground is the normal procedure for producing masses of new shoots and lush colourful foliage. It looks savage - but it works. The pruned shrub will normally regain its normal - size prior to pruning - in the growing season Spring to Autumn.