Foxtel has revealed the source and the scale of the "technical glitches" that prevented thousands of subscribers from watching the season-seven debut of Game of Thrones on Monday night.

In a statement issued just before 3pm on Tuesday, the company apologised "unreservedly" to customers affected by the issue, while promising to "do everything possible to ensure that this does not occur again".

The technical issues, which were first reported around 8pm on Monday night, were the result of a "surge in demand" for its new streaming service Foxtel Now, the company said.

"The number of Australians subscribing to … Foxtel Now leapt by 40 per cent in the 48 hours prior to [Monday] night's screening of episode one. The combination of new Foxtel Now customers signing up and existing customers upgrading to get the Drama Pack so they could watch the show put unprecedented pressure on our technical operations."