Work and Income pay you for the week that has just passed (in other words, “one week in arrears”). You have to declare your income for the week just been (often on a Friday before the working week is over).
You might have to predict how much you’ve earned before you actually get paid. If you under-predict your earnings, you could end up with a debt to Work and Income. If you overpredict your earnings, you can correct it later and Work and Income will pay you a top up the following pay week.