It is about 6.5 x 5 centimetres and semi transparent. It has quite a tough skin and is hollow inside. Like a tight clear tooth shaped balloon. I have put a coin next to it for you to see the size.
It's a swim bladder, probably a porcupinefish swim bladder. They can be found washed ashore still inflated. They've been described as "molar shaped". (Picture from Australian Museum.)
It's hard to tell exactly, but if I had to bet I'd say it's a small part of an algae called Neptune's necklace:
Some species are native to New Zealand and Australia. Where two "branches" split, the shape you found can be produced.
Looks like they're also found in your area: http://bie.ala.org.au/species/NZOR-4-7414