Got this via email. One needs to buy $10 1GB plan before May 31st with a targeted and unique coupon code. Then they'll get their first 3 renewals of UNLIMITED 1GB for free ($10/28 days thereafter). May work in conjunction with regular Amaysim referral offer.
Edit: It is part of contest to win iPad Air 64GB. Coupon code is specific to individual.
Special terms and conditions - amaysim mobile - Get the Unlimited 1GB or 2.5GB free for the initial purchase and the next 2 renewals
Promotion Period - 12.01am on 1 May 2019 to 11.59pm on 31 May 2019
Eligible Plan - UNLIMITED 1GB ($10/28 days), UNLIMITED 2.5GB ($20/28 days)
Eligible Customers - Customers who place a new order for an Eligible Plan at amaysim.com.au and enter the applicable promo code when signing up; and enter an approved payment method when prompted, are eligible for the Promotion Benefits.
Promotion Benefits - Eligible Customers will receive their initial purchase and the next 2 consecutive renewals of an Eligible Plan free. The promotional code is valid for redemption until 11.59pm (Sydney time) on 31 May 2019.
I got a similar email but different code, so think it's unique.