IRD have changed their policy on when payments will be considered to have been received on time. Payments made by post are now treated as made on the day Inland Revenue receives them; the date of posting is irrelevant. It’s therefore up to you to make sure you post your cheques in good time to reach IRD on time. There’s no guarantee that a payment posted on the 18th will reach Inland Revenue by the 20th.
If you’re sending a post-dated cheque, Inland Revenue will not bank it until the date specified. So even though it’s physically received before the due date, it will still be treated as received late if the specified date is after the due date. You can also make payments in person, either at an Inland Revenue office or at a Westpac branch (note Westpac no longer accept cheques for tax payments) as long as you do so before close of business on the due date. Now might be a good time to think about making your payments online, if you don’t already.