Taylor Schilling as Piper Chapman in Orange is the New Black, among the offerings on Netflix.

The high take-up of internet television is slowing some broadband connections to a crawl in the evening.

Wellington researcher TrueNet said broadband speeds appeared to be slowing in the evening as a result of the growing popularity of new "on-demand" television services such as Lightbox, Neon and Netflix.

Network operator Chorus confirmed it had seen an uptick in internet traffic over the past few weeks and that demand for capacity was growing at a faster rate than it had anticipated.

Ross Giblin Steve Galyer says it's 'irresponsible' for internet providers to offer 'unlimited broadband'.

"We are still digging into the network data to truly understand the impact," spokesman Nathan Beaumont said.

TrueNet said it had observed a sudden deterioration between 8pm and 10pm, beginning in March.

During the previous six months most internet services had been humming along well at peak times, it said.

The drop in the peak-time performance of Vodafone's cable network, which is available in Wellington and Christchurch, had been "pronounced", the researcher said in a report.

"The market for television and movies on the internet ('over-the-top' video) suddenly took off this year, and March appears to be the month that demand started to impact supply.

"Both Vodafone and Slingshot had severe copper broadband performance reductions during the peak demand hours of 8pm to 10pm, while Snap and Spark also dropped significantly," Truenet said.

Wainuiomata businessman Steve Galyer had expressed nervousness in January about what the local launch of Netflix might mean for his shaky broadband service, and said his fears appeared to have been realised.

Galyer, who runs a home-based forestry software business, said he had been forced to stop work during the April school holidays as his internet service became unusable and had also noticed a big impact during the evenings.

"It is much worse than it was during the Christmas school holidays. I literally can't work. I don't dare go in and start altering a customer's database, because I don't know whether I am going to be able to complete the job," he said.

TrueNet director John Butt said that while he couldn't prove the nationwide peak-time performance drop was due to the rising take-up of internet television, it was the obvious culprit.

"It lines up with the timing of Netflix and other video options coming on stream."

Slingshot had previously seen performance drops in the evening which might reflect it offering access to Netflix' United States service through Global Mode, he said.

Butt was concerned the peak-time snarl could get worse, given it was still early days for internet television.

"I assume the number of people buying 'over-the-top' video services is still quite small. What happens when we get into the middle of the adoption curve when the numbers are increasing dramatically?"

Both copper and cable broadband services appeared to be affected but households that had switched to ultrafast broadband were largely immune, he said.

"If you are on cable it is shared with neighbours down a street. If any of those sign up for Netflix, they are going to affect everybody."

Galyer said he could see broadband speeds drop off when others in his community turned on the internet. I could have 5 megabits in the morning and two minutes later it would drop down to 1Mbps.

"What that tells me is there is one or two people coming online and sucking the guts out of it."

Vodafone chief executive Russell Stanners wrote to Galyer last week acknowledging his complaint. Vodafone was willing to keep looking at options but did not have a solution, he said.

"What is clear to me is that the [copper] network is working as designed, it is not a fault and until such time as Chorus decides to upgrade the network, which they may or may not do, there is no immediate solution to your network slowdown issues," Stanners said in the letter.

Galyer said it was "irresponsible" of Vodafone and other internet providers to offer unlimited data on a network that was already "creaking at the seams".

Beaumont said there were many factors that could impact "end user experience".

Not all retail providers were caching Netflix programming locally in New Zealand "which means they are long hauling content from the US, with performance impacted by international capacity", he said.