Per the page, details below:
"For a limited time only.
Our Annual Gift Card Sale Is Here – Enjoy 20% Off for a Limited Time
Give the gift of good times – now with even better value. For a limited time only, enjoy 20% off all Merivale Gift Cards when purchased online.
Whether it’s a long lunch, date night, cocktails with friends or an overnight stay, a Merivale Gift Card unlocks access to dining, drinks and entertainment across all our venues.
Offer Details:
• 20% off all online gift card purchases (e-Gift Cards and physical cards)
• Available exclusively via giftcards.merivale.com
• Valid at all Merivale venues, bottle shops and accommodation
• Gift cards are not valid at stadium outlets (Allianz and Sydney Cricket Ground) or for private events
• Full terms and conditions apply
Hurry – this offer is only available for a short time."
It's unclear when the end date is but I would assume it is an EOFY deal.