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
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
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
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
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.
WA
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.
VIC
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.