It's notoriously tricky to rate the speed of an ISP's DSL service. Each customer's experience varies - often wildly - depending on physical factors such as how close they are to their local phone exchange (for DSL bandwidth degrades with distance; you can be 6km away, but people 2km and closer will be surfing much faster), and the age and quality of the copper phone lines running up to your house (every bend in the road, or other kink in the line, causes interference, again sapping your broadband experience).

Then there are environmental variables, such as the time of day (at peak times, more people have to share the same bandwidth), or whether your TV decoder, home-office fax or poorly-configured modem is crimping your line at any given moment.

Epitiro, an independent ISP testing company from the UK that set up shop in New Zealand earlier this year, seeks to eliminate as many variables as possible.

The company's NZ MD, Mike Cranna, says that for Epitiro's first NZ survey, it signed up to premium plans on TelstraClear, Slingshot, Orcon, Xtra and iHug.

It then used five PCs to access each of the five ISPs simultaneously from a CBD location in Auckland, with the same exercise mirrored in Wellington and Christchurch. The five PCs in each city accessed each of the five ISPs every 15 minutes, 24/7, from July 20 to September 10.

Cranna, who claims his survey is the first "definitive" broadband analysis carried out here, says TelstraClear's DSL service (as opposed to its Wellington and Christchurch-only cable broadband service) easily came out on top - and in fact rated very well by international standards, clocking a higher average speed than UK providers. Slingshot was second, Orcon third.

TelstraClear came out top because it's the provider that's gone furtherest in creating its own, Telecom-independent network, Cranna says.

"TelstraClear has a relatively new network which extends close to the customer and far into the global internet. Consequently, without needing to use Telecom's older and more contended network, they are able to offer a better service"

And by Epitero's analysis, Telecom's older network is not a good place for Orcon, iHug and Slingshot to be. ”The data shows that there are ongoing issues with Telecom's network, which is dragging down the performance of those ISPs piggy-backing on it", he says.

But not all problems can be laid at Telecom's door: "Our data shows some ISPs are not managing their traffic as efficiently as others, or purchasing enough international bandwidth to cope with peak demand. We see packet loss or response times go through the roof once demand increases," Cranna says.

Hopefully the overall situation's going to improve, rapidly, as local loop unbundling allows Orcon, iHug and others to install more of their own DSL gear inside Telecom's exchanges (as I write, they are only inside two, in the Auckland suburbs of Ponsonby and Glenfield).