As stated on their Facebook page, get $25 gift which will offset the normal signup fee and SIM delivery cost.
Feel free to use a referral link on
https://www.ozbargain.com.au/wiki/spintel_referral_links
and get an additional $10 or more credit
More info on referral here
https://www.spintel.net.au/referafriend
Had problems with spin tel not honouring the referral credit and disconnecting from them was a massive hassle that I had to cancel the bank card to stop them charging me even with confirmation of disconnection.
Also there is a 25$ setup fee.
IMO go with aldi