In other markets, call roaming rates would be reduced by up to 50 per cent or remain broadly similar to current rates.
For prepay customers, data roaming rates will be reduced by up to 88 per cent and call roaming charges by up to 45 per cent.
Telecom chief executive Simon Moutter said the flat rate would make it much simpler for customers to understand data roaming costs and would give them the freedom to make use of their smartphones in the same way they would at home.
"We know data roaming charges have been a pain-point for customers - personally, it was a real bugbear of mine as a customer before I joined Telecom earlier this year. A flat fee provides certainty and puts an end to concerns about nasty bill shocks on your return home.
"It's also much simpler to follow than any usage-based system - as most customers don't really know how quickly their phone apps will chew through 1MB, 10MB or 100MB."