iiNet and Internode are the latest internet service providers to offer compensation to customers over misleading marketing about NBN speeds.
It follows parent company TPG's December promise to compensate almost 8000 of its own NBN customers after selling them onto NBN speed tiers they were unable to achieve.
The ACCC had threatened to take NBN service providers to court for misleading NBN speed marketing.
iiNet and Internode today said they would compensate 8000 and 3000 NBN customers respectively.
The ACCC said both companies had likely misled customers by promoting NBN plans with maximum speeds that could not be delivered.
Both had advertised their highest-tier FTTN and FTTB plans as delivering speeds of up to 100/40 Mbps.
The ACCC said 7621 customers on iiNet's FTTN 100/40 Mbps plan could not achieve the advertised speeds - and 1925 of those users couldn't even get 50/20 Mbps speeds.
Just over 1700 users of Internode's 100/40 Mbps FTTN plan similarly weren't able to reach those speeds, with 479 of those users similarly unable to attain 50/20 Mbps.
The regulator said a smaller number of customers on FTTB plans were also affected.
iiNet and Internode are now offering affected customers the ability to either move to a lower speed tier plan, or exit their existing plan free of charge. Both of those options also come with a refund, the ACCC said.
Affected customers will be contacted by April 27.
“Internet service providers must provide accurate information to customers about the speeds they can access on the NBN," ACCC commissioner Sarah Court said in a statement.
"Many customers could not reach the maximum speeds advertised by iiNet and Internode because their NBN connection was not capable of delivering it. Some customers couldn’t even receive the maximum limit of lower speed plans.
“Fixing misleading claims about internet speeds during the transition to the NBN is an enforcement priority for the ACCC and we strongly urge other providers to act quickly to ensure their advertising is accurate."