The diuretic can be attributed to the caffeine present in tea and coffee. Caffeine increases diuretic action on the kidneys, increasing urinary volume and sodium extraction as a result of a decrease in the tubular re-absorption of sodium and water. Coffee contains a higher content of caffeine compared to tea. Research has shown that a 170ml (6-oz) serving of tea contains, on average 34mg of caffeine in comparison to 99 mg of caffeine in 170 ml serving of brewed coffee.
As a result the diuretic effect of coffee is greater compared to tea.