Brands to be blocked from Australia include Syndicate Casino, Fast Pay Casino, BitStarz and King Billy Casino.
The ACMA said it had received numerous complaints about the 18 websites, and following investigation found them to be operating in breach of the Interactive Gambling Act 2001.
Website blocking is one of a range of enforcement options the ACMA uses to combat unlicensed gambling in the jurisdiction.
Since the authority made its first blocking request in November 2019, 222 gambling websites have been blocked. This included blocking orders issued in August, October and November 2020.
In addition, the ACMA said over 100 unlicensed operators have pulled out of the Australian market since the authority started enforcing new offshore gambling rules in 2017.
In November, the ACMA reminded all the licensed operators in Australia to comply with gambling advertising rules after highlighting inconsistencies in industry interpretations of the controls.
The authority has monitored the effectiveness of advertising rules since 2018 when they were brought in. Although it had not identified any major concerns, it did uncover inconsistencies in the ways gambling companies interpreted the rules.