T&Cs as follows - it seems to be targeted and I referred to a previous post and comment below.
Terms and Conditions
$20 off is available on purchases of $100 or more (before delivery or click and collect cost) and is available on online at briscoes.co.nz or in any Briscoes store. To redeem this offer, you must mention or show this email in store prior to payment or be logged into your account on the website and enter coupon code TWENTYOFF at checkout. If you don't have an account please create one or click forgot password to reset your account. Offer available for 30 days from when this email is received. Code must be applied at checkout and doesn't apply to previous purchases. Discounts exclude the purchase of gift vouchers. Personal shoppers only, trade not supplied. No phone or mail orders, no rain checks. If a refund is requested, we will refund you on a pro rata basis across all items that was purchased in the order. Offer cannot be used in conjunction with another coupon code.
EDIT: Quoting @bigcheese (forgive me if you're not cool with it, let me know and I'll remove it) here on a past post:
It is targeted - I get these all the time across my multiple Briscoes accounts.
This one is sent when the account hasn't been used for a while and they are trying to entice you to spend.
Unfortunately only works on those selected accounts that meet their criteria so for most people it won't work.
Someone might have some luck trying it instore, but no guarantees it will work.