COE 2021, page-44

"Under the Transition Agreement, APA will make contributions to the cost of certain back-up supply arrangements in instances of production shortfalls. Cooper Energy will earn a comparable net cash margin as if all the gas had been processed at the OGPP, and a superior net cash margin to that which would occur through supply at anticipated near-term spot gas prices."

I interpret this as

Say Required supply =54TJ/day
Production from Orbost =24TJ/day
Sales price = $7/TJ
Margin = $3/TJ

The shortfall = 30TJ/day (54 required less 24)

So COE needs to spot purchase 30TJ
Spot price say = $5/TJ
APA contributes to that purchase (it's silent on how much), say 50%

Cost to COE to purchase is $2.50/TJ
APA also pay COE the comparable margin of $3/TJ from above

So basically COE earn
$3/TJ on Orbost processed gas of 24TJ/day
$0.5/TJ on back up purchased gas of 30TJ/day in this example