Damn it I just used my 15GB 1 year code when it was 50% off.

I wonder if support would "upgrade" me to that deal ;)?

Damn, thats tempting.  

Having never really looked into Kogan before, do these offers come up regularly?  So in say 12 months time, is there a chance a similar offer/voucher comes up, or can you stack if they come up earlier?

They have very attrative offers like this several times per year.  I buy my kids 365 day vouchers toward the end of the year, typically on Black Friday.

The catch is that you are locked in for 1 year.  Any change of plan at all results in the remaining portion of your current plan being lost.

https://help.koganmobile.co.nz/hc/en-us/articles/360032549352-Why-do-I-have-to-wait-until-my-Prepay-Plan-Add-On-expires-before-I-can-get-another-one- 

I'm hoping the recent Aussie student deal makes an appearance here:

Kogan Prepaid Mobile 30 Days 30GB SIM $0.01 Delivered @ Kogan