First establish if the source is your own plumbing or a local issue.
If it is the cold water that is brown, check with your water provider to see if they have been doing any water supply works in your area. If it is caused by works, the problem will soon settle. Run the tap until the water turns clean before drinking it.
Contact Sydney Water
By phone: 132 090
Contact Hunter Water
By phone: 1300 657 657
If it is a plumbing issue, brown water may be caused by rusty iron piping. This will need replacing. It might also result from corrosion inside an old hot water system - if it is, the brown water will come only from the hot water tap. In this case, get the unit checked by a plumber. It may need replacing.