Vodafone argued its cable networks were a good enough substitute for fibre-optic UFB but the Government disagreed.

The competition watchdog is investigating Vodafone's marketing of its cable networks in Wellington and Christchurch.

The investigation has been prompted by concerns consumers may be confusing the cable networks with the government-backed ultrafast broadband (UFB) network.

A Commerce Commission spokesman said it had received several complaints and its investigation was at an early stage.

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Vodafone rebranded its cable networks as "FibreX" last year following a technology upgrade that allowed them to provide maximum download speeds of close to a gigabit, and argued they could provide a similar experience to UFB.

It also describes the cable networks as providing ultrafast fast broadband.

The cable networks use HFC (hybrid fibre-coaxial technology) which is widely regarded as superior to standard copper broadband but which does not provide fibre-optic cabling all the way to the home.

Vodafone consumer director Matt Williams said the "X" in FibreX indicated to consumers that the service was not pure fibre.

"Earlier this year the Advertising Standards Authority ruled that Vodafone's FibreX advertising was not misleading. At present we don't have plans to change the name or marketing," he said.

Checks on some addresses in Wellington indicated that if customers currently served by cable keyed their address into Vodafone's website, it would not reveal whether UFB was available, instead directing them to FibreX as their "fastest" available option.

Williams confirmed Vodafone was promoting the cable networks on its website. "But if a customer specifically asks for Vodafone Fibre we will give them that," he said.

Vodafone and before it TelstraClear – which previously owned the cable networks – had lobbied the Government not to lay UFB in areas covered by the cable networks arguing that was a waste of money.

But the appeal was rejected by then communications minister Amy Adams who said Vodafone was "pursuing its own commercial interests".

Williams said FibreX offered customers a fast broadband experience "consistent with the other fibre networks".

If any FibreX customers felt it was not achieving that, they could get in "and we will sort it out", he said.