Just under a third of homes in Chorus zones with access to UFB have signed up for the fibre-optic broadband service.

Ultrafast broadband is coming to more than another 200,000 homes, but doubts are already being expressed that the expansion of the network isn't quite ambitious enough.

Another 423,000 people will be able get ultrafast broadband (UFB) by the end of 2024 as a result of a long-awaited decision to expand the network.

Prime Minister Bill English said UFB would be extended to more than 151 additional towns, on top of the 33 cities that are already getting the service.

Maps of the towns that will now get fibre are here: North Island South Island

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The expansion will mean UFB will be available to "up to 85 per cent" of the population, up from the 75 per cent coverage that is planned to be delivered by 2020.

Telecommunications Users Association chief executive Craig Young said the Government needed to be congratulated but it wanted a "fibre-like" service to all of the country.

"85 per cent is another good step along the way but we will continue to argue for the other 15 per cent to get some sort of service that is equivalent," he said.

At the moment, there is no other technology that is equivalent to UFB, he added.

The new coverage target is higher than the 80 per cent goal that former communications minister Amy Adams set for UFB2 when she first announced there would be a second leg to the UFB scheme in 2014.

However, it is below the 90 per cent figure that industry sources said could be achieved without pushing the per-premise cost of the roll-out to unrealistic levels.

InternetNZ chief executive Jordan Carter said the new target was impressive and more than it had been expecting, but agreed it was "not the end of the road".

"Whether there needs to be a 'UFB3' or not . . . we need to digest the areas that are being covered and talk with people about the economics beyond that."

Communications Minister Simon Bridges would not comment directly on whether there might be a UFB3 down the track, or whether the new footprint of the network was likely to be final.

"The Government is firmly focused on the future broadband initiatives such as phase two of the Rural Broadband Initiative (RBI2) and the Mobile Black Spot Fund (MBSF) which will deliver faster internet to areas not covered by UFB1 and 2," he said.

Raetihi with a population of 1000, but not @NashParkVillage with annual tourist numbers far exceeding that as the gateway to Tongariro NP🤔 https://t.co/7GE8pl9xI6 — Louis van Wyk (@LouisvanWyk) January 25, 2017

The cost of the extra coverage would be $300 million, the Government said.

Adams had set a cap of $210m on the expansion, but Bridges said the Government had been able to allocate more money thanks to an underspend on the original UFB roll-out, which had been more efficient than expected.

Labour communications spokeswoman Clare Curran said she was suspicious of that explanation and would be seeking more information.

"If I was living in one of the 151 towns I would momentarily feel pleased but then I'd look at the timeline – six years."

The four existing UFB network companies, Chorus, Enable, Ultrafast Fibre and Northpower will pick up the job of building out the network, with Chorus getting the bulk of the work.

Chorus said its contract would see it extend UFB to another 200,000 homes.

Curran forecast electricity lines companies might seek to take advantage of new environmental legislation that will make it easier for them to string fibre-optic cables on powerlines and get the job done sooner in some areas.

Bridges said rolling out faster, more reliable internet was a vital part of developing a productive and competitive economy. It would put New Zealand "among the leaders in the OECD for access to fibre", he said.