The Commerce Commission has started an investigation into Vodafone's "unlimited" mobile plan launched in July which has a number of restrictions listed on the plan's promo page.
The NZ Herald reports the question is whether the "limited" factors are sufficiently prominent.
The news outlet's online report says the ComCom refused any comment while its investigation was open.
The restrictions are listed on Vodafone's Unlimited Mobile promo page, but only after you click a link labelled "Important things to know".
Vodafone's "Unlimited Mobile" plan is $79.99 a month and NZ Herald says like "unlimited" plans launched earlier by rivals Spark and 2degrees, it has a number of limits.
These include speed being reduced from 4G to 3G if a customer downloads more than 22GB of mobile data within a month, and streaming video restricted to standard definition.
A spokeswoman for Vodafone said the company is "working through the details with the Commission and will co-operate fully with their investigation".
The regulator last month laid 10 charges against Vodafone under the Fair Trading Act for billing beyond the date of some customers' notice period.
In another legal action, the commission is targeting Vodafone's Fibre X marketing campaign.