This is supposedly a Gumtree-Sendle offer, but they don't seem to link to each other, so you can also send non-Gumtree parcel for $1 (EBay sale, present, etc) if you sign up Sendle as a new customer from their promotion link. Typical cost for 5kg/20L via Sendle Premium is $14.95, with a surcharge of $13 for regional areas.
Available for new customers only for parcels to domestic destinations, maximum size 5kg/20L.
The promotion begins at 00:01 August 10th 2020 and ends at 11:59 September 20th AEST (“Promotion Period”).
To claim your $1 shipping offer, please add a credit card to your account.
Once you've added a credit card, you'll see the discounted $1 shipping applied to your order.
P.S. They are a front end company using Couriers Please and Fastway couriers (like third party mobile phone service providers reselling service on the main mobile networks). I find that they are no better or worse than other companies (Aust Post , TNT etc). Once in a while, there'd be a stuff up just like the others.
I would never send an item on Gumtree unless the payment was a direct deposit