What is a marginal tax rate?
Your marginal tax rate is the rate of income tax you pay on your next dollar of income.
New Zealand taxes each slice of income at its own rate: 10.5% up to $15,600, 17.5% to $53,500, 30% to $78,100, 33% to $180,000 and 39% above that. A pay rise only lifts the tax on the extra income, not on everything you earn.
Your marginal rate affects the tax on interest and on FIF income, while PIE income is capped at 28%.