Some brands seem to have more elastic urethane than others.
Find one that is the least elastic.
I do remember having that problem when I borrowed a Balin brand leash off an Aussie friend a few years back.
Also had the same problem with FCS brand leashes, a few years back.
They were real stretchy, with boards rocketing back at you.
Da Kine brand leashes, have been a very reliable choice for us here in the USA.
Surf More XM brand is another good choice we have.
Not sure of all the different brands you have to choose from now there in OZ.
6-8'rs seem best for most purposes.
Around here 8'rs often turn into 12'rs after one session
And always try to cover your head with your hands if possible during and surfacing after a wipe-out.