I had a look & the coupon CELEBRATE15 does work with buying Kogan.com credit via their ebay store.
$50 Credit - $42.5
http://www.ebay.com.au/itm/50-Kogan-Credit-/171455471196?pt=…
$100 Credit - $85
http://www.ebay.com.au/itm/100-Kogan-Credit-/161419026084?pt…
$500 Credit - $425
http://www.ebay.com.au/itm/Kogan-500-Credit-/161421014505?pt…
$600 Credit - $510
http://www.ebay.com.au/itm/Kogan-600-Credit-/171458025253?pt…
Obviously the bigger the credit purchase you buy, the bigger the discount. Enjoy.
Someone astute in the ebay thread raised the point that they might come in multiples of $100 and as such you would only be able to use $100 per transaction. Can anyone shed some light on this?