Found on my Amex dashboard page.

Save offer to your eligible Card by 31/05/2018 and spend $50 or more, in one or more transactions, online at www.momentumenergy.com.au or via phone on 1300 662 778 by 30/09/2018 to get a $25 credit.
First 15,000 Cards to save the offer.

T&C’s follow-
OFFER TERMS

  • You must save the offer to your eligible Card by 31/05/2018, and make payment by 30/09/2018 to be eligible for the offer.
  • Valid for payments made to Momentum Energy via Direct Debit, online, or via phone on 1300 662 778.
  • Offer is limited to one credit per Card to which the offer is saved and only spend on this Card counts towards the Offer.
  • Excludes transactions where you do not spend directly with your Card to which the offer is saved, at Momentum Energy at www.momentumenergy.com.au. Offer valid at Australian website only.
  • Excludes transactions made through a third party establishment or payment processor such as BPAY or Australia Post.
  • Credit is not redeemable for cash or other payment form.
  • Credit should appear on your billing statement within 5 business days from qualifying spend but may take up to 90 days from the offer end date.
  • Credit will not be applied to your Card account if your Card has been suspended or cancelled.
  • Credit may be reversed if your qualifying purchase is refunded or cancelled.

Updated link thanks Forumninja