Found this whilst browsing the IKEA site. It doesn't seem to be on the T&Cs page yet but the tile appears on store specific pages. T&Cs
IKEA Family member offer:
Valid Friday 1 Nov - Monday 11 Nov.
Shop in-store and receive an IKEA voucher to save on your next visit between 12 Nov - 31 Dec 2019.
Spend between $100 to $249.99 and receive a $20 voucher.
Spend over $250 and receive a $50 voucher.
Promo tile only shows on these four store pages: Available at all stores
Adelaide (https://www.ikea.com/au/en/store/adelaide/)
Canberra (https://www.ikea.com/au/en/store/canberra/)
Logan (https://www.ikea.com/au/en/store/logan/)
Marsden Park (https://www.ikea.com/au/en/store/marsden_park/)
North Lakes (https://www.ikea.com/au/en/store/north_lakes/)
Perth (https://www.ikea.com/au/en/store/perth/)
Rhodes (https://www.ikea.com/au/en/store/rhodes/)
Richmond (https://www.ikea.com/au/en/store/richmond/)
Springvale (https://www.ikea.com/au/en/store/springvale/)
Tempe (https://www.ikea.com/au/en/store/tempe/)
NOTE:
* T&Cs state $10 voucher per $100-$249.99 spend. However, email sent out to members confirms that it is a $20 voucher.
* Your voucher will be printed on your receipt, so make sure you hang onto it!
So the best amount to spend is either $100 or $250 for 20% bonus.