No, they are legal, and considered toys. Please see the document below:
As of December 2017, the Department of Home Affairs in Australia has re-classified gel blasters as a toy and not a firearm.

QLD
100% legal to purchase from reputable businesses, legal to own and no license is needed.
SA
Classified as a toy however some of the gel blasters may be considered as an imitation firearm. It's advised that customers need to consult with their local law enforcement.
NT
Classified as a toy however some of the gel blasters may be considered as an imitation firearm. It's advised that customers need to consult with their local law enforcement.
NSW
Classified as a toy however some of the gel blasters may be considered as an imitation firearm. It's advised that customers need to consult with their local law enforcement. The NSW fact sheet regarding imitation firearms clearly states that "An imitation firearm does not include anything that is produced and identified as a children's toy". Please see link the fact sheet
https://www.police.nsw.gov.au/__data/assets/pdf_file/0012/13323/FACT_SHEET_Things_Declared_not_to_be_Firearms_V1.3_Sept_2017.pdf
TAS
100% legal to purchase from reputable businesses, legal to own and no license is needed. We recommend making the gel blaster easily identifiable as a toy.
WA
100% legal to purchase from reputable businesses, legal to own and no license is needed.
VIC
Classified as a toy however some of the gel blasters may be considered as an imitation firearm. It's advised that customers need to consult with their local law enforcement.