Optus, Australia's most expensive network for data roaming, says it will start proactively warning customers when they start clocking up big bills overseas.

Monthly-billed customers will get an SMS alert when they first download any data on an overseas mobile network, warning them of the high costs of global roaming (a staggering $20 per megabyte on Optus).

The SMS will prompt them to disable or stop using data roaming on their device if they want to avoid further charges.

However, if they choose to keep using data roaming at Optus' amazingly high rates, the telco will SMS them to let them know each time they've burned through a block of 15MB of data -- $300 worth.

Optus Marketing Director Gavin Williams says, "We believe it will lead to fewer customers coming home to unexpectedly high mobile bills or at least make customers more aware of how much it will cost them if they do choose to use data roaming.”

Williams also again promised that Optus would reduce data roaming rates at some point in the future.

"Improved data roaming rates for popular destinations are just some of the measures we are looking at to provide Optus customers with a better experience,” he said.

Australian Business Traveller's advice on using global roaming with a major Australian telco?

Don't do it.

It's now so cheap and easy to buy a prepaid SIM card at your destination, or use a specialised global roaming SIM card with massively reduced rates that using global roaming is simply a way of spending a lot of money quickly.