Could be Rubberweld Marine self amalgamating tape

Self amalgamating tape.
Bunnings have it in the same section as electrical tape.
I think it's actually silicone.

I think you might be referring to 3M type 23 self amalgamating rubber tape.
3M had the patent on it for years but it ran out a few years back so now anybody can make it, and they do.
There are now several brands but I think the original 3M type 23 is still the best.
It's a bit expensive though.
It's the ducks nuts for all sorts of stuff, although it's original specification was for electrical insulation tape.
It has a rating of about 5000 volts per mm. Not that this would be of much use to you I spose.

Most of the Bunnings staff have never heard of it and generally direct you to the wrong place.
I think you can also get it from some auto parts shops, like SuperCheap auto parts or similar.
It will cost about $15 for a small roll.