From the Westpac site:
Save up to 25,000 KrisFlyer miles after your first year
After every year, you'll be rewarded with a special thank you for your loyalty to your Singapore Airlines Westpac Cards - for example, after you've had your card for one year, you'll receive a renewal gift voucher1 that allows you to enjoy a 50% discount (up to a maximum of 25,000 KrisFlyer miles for Platinum, and 15,000 KrisFlyer miles for Gold) off an economy saver redemption ticket or upgrade.2
Now I draw your attention to that...economy saver. Though oddly the term economy saver is not used in the fine print.
2. The voucher (25,000 miles for Platinum Cards, 15,000 miles for Gold Cards) is only available for one-time use for one ticket or upgrade on Singapore Airlines for either the Principal cardmember or their KrisFlyer redemption nominee. The 50% discount is only applicable to the KrisFlyer miles required for the redemption of the award ticket or upgrade. The 50% discount is not applicable to miles used to pay for KrisFlyer service fees or to fees, taxes and surcharges chargeable for the flight. Offer is considered redeemed even if less than 25,000 KrisFlyer miles (for Platinum card) and 15,000 KrisFlyer miles (for Gold card) are used. Excess KrisFlyer miles will not be credited to the Principal cardmember's KrisFlyer account. Upgrade redemption award can be utilised on a commercial flight (in upgradeable class) purchased by Principal cardholder for travel on Singapore Airlines operated flights only. The voucher must be redeemed within 6 months of it being generated.
Offer is subject to seat availability and cannot be combined with any other discounts or promotions. Offer is to be used in conjunction with KrisFlyer miles in cardholder's account. KrisFlyer terms and conditions apply. Westpac and Singapore Airlines reserve the right to change the terms and conditions at any time without prior notice.
So there are a few conditions on it's use and it is not clear (see highlighted text in blue) whether you can still get the 15% on-line booking discount..
These conditions reduce its value to me as to get full value you have to fly on the one trip witha redemption worth 50K or more in economy. My current SQ redemption plans are all in J.