San Diego gets an average of 10 inches of rain a year. Most people don't think that's worth catching. But a 1000 square foot roof can catch 600 gallons of water in 1 inch of rain. That can add up pretty quickly!
Check out this Sydney Guide for selecting a tank.
Considerations for water tanks are material: plastic, metal, ferrocement. Don't use white tanks as they will grow algae in the sun.
The amount of filtration you do before the water gets into the tank will determine the amount of maintenance you have to do. Simple filtration includes a Leaf Eater and Downspout Diverter to divert the first flush of water which is usually the dirties from the tank. According to the city of Sydney you need to check your tank for sludge at least every two to three years.
It is ideal to have your tank at a high point in your landscape in order to avoid a pump. If you do intend to use drip irrigation, you may need to use a pump to get ideal efficiency.
There are several tank materials to choose from including plastic, metal, concrete, and fiberglass. Plastic and metal are relatively lightweight and easy to move into position. You want to choose a dark, thick plastic to avoid algae growth and withstand UV rays. You may need to line a metal tank to prevent corrosion. Concrete can be made to shape using ferrocement techniques discussed in a book by Art Ludwig.
There are two types of tank systems: a wet and a dry system. In a dry system the water is all gravity fed from the source (the roof) into the tank. In a wet system, you can have the tank some distance from the house and put the pipes underground and bring them back up into the tank. As long as the inlet to the tank is below the gutter entrance, the water will flow into the tank. The issue with this system is that you will always have standing water in the pipes. If you have freezing temperatures, this could cause pipes to break. Also, you need to protect both the entrance and outlet against mosquitos.
Always plan an overflow. If your tank fills, where will the extra water go? It's great if you can send it into a greywater basin and push the built up salts down through your soil.