If there's one thing I've learnt in 20 years of landscaping it is that New Zealand is a wet place. I've actually learnt to quite enjoy working in the wet. Besides making my own disposition contented, the other outcome of working outdoors in a comparatively wet country is you get to know the soil conditions and how different soils perform under different circumstances.
Establishing plants in very wet heavy soils can prove very difficult, so a two-pronged approach may be required. The first prong is colonisation - plants to help get the water moving and oxidise the soil. A colonising species planted in a wet area drinks up the water, provides shelter and breaks up heavy soils. The improved soil conditions then create a healthy habitat for the desirable plants we would prefer. In wet soils, natives such as cordyline australis, and pseudopanax arboreus work wonders or for some lively colour, canna lily and ornamental bananas make good colonisers.
But in really extreme situations, drainage may be the only option. Creating effective drainage to control water is actually quite a skill because balance is required. A nice dry winter lawn can become a liability in summer when it will need constant watering to stop it drying off too much.
The first step is to find out where excessive water is flowing from. As water flows down it should be fairly easy to see where "upstream" the water is starting. The best thing is to control the water before it becomes a problem. Any retaining above a lawn area provides the perfect opportunity to do this. Ideally you want to redistribute the water around your section rather than running it off-site, which creates environmental issues in our streams, estuaries and harbours.
I don't recommend running a drain directly through a lawn if at all possible. It is best to slope the lawn to create some run-off, place any drainage at the highest point to divert excess moisture and concentrate on improving the friability of the soil conditions.
For drainage to be effective it must be constructed properly. The critical steps are to use the right aggregate and make sure the pipe being used to get the water away is sloping down and has somewhere for the water to clear at the bottom (obvious, I know, but believe me I've seen the results when that doesn't happen). From then on you need to make every effort to keep the drain clear of silt. For a standard garden drain, I use S A P 20 as the aggregate, which is a scoria with aggregate sizes of 20mm. Scoria is ideal because it will never actually bind together (unlike builders' mix for instance) which means water will always be able to travel through it. Once the drain has started collecting water there needs to be a means for it to escape which is where the pipe comes in. Using perforated flexible pipe like Novaflo coil allows the water to travel away from the drain. Silt build-up can very quickly render a drain ineffective so to prevent this before it happens the whole drain is wrapped in filter fabric. I always cover the pipe in filter sock which is like a stocking that stops silt building up in the pipe. This is a cloth (now often sold as weed mat) which is placed directly into the drain on the bare soil.
Install the coil on a bed of scoria around 100mm deep, sloping downward so water runs off. Place more scoria over the coil, then fold the ends of the filter cloth over the top of the scoria.
Finally I usually use river boulders on top of the scoria as they look good in most gardening situations and keep the drain clear of litter.
Once you've controlled the water you are able to divert it around the lawn and away into a less-frequented part of the garden. You can dig a shallow sunken garden bed and plant plants that do well in water, like swamp iris. In a vege garden, raising the beds and taking a no-dig gardening approach will take care of heavy soil. Good landscape design is the best place to start; you'll be amazed how much water you can get rid of just through clever plant selection.