The telco is working to restore mobile services to a huge portion of southern NSW after not one but two fibre cables 130 kilometres apart were cut, wreaking havoc on dozens of mobile sites on the local network.

Telstra's southern region general manager, Chris Taylor, confirmed staff were inspecting the cables on Wednesday morning after the series of unfortunate events.

It began on Tuesday evening when a back-up line was cut near Temora, which was then exacerbated when main one was damaged at Adelong several hours later.

"At about 5.30pm [Tuesday] we had a fibre cut near Temora that didn't affect any services but it was our redundant, or back-up, fibre for most of the infrastructure in the area," Mr Taylor said.

"Then a few hours later we've had another cable cut near Adelong, which has taken the main line ... that's caused an impact on 44 mobile sites."

The damage was caused "by independent construction in the area", Telstra said on its website.

The outage has affected a number of Telstra services in the MIA Griffith, Yenda, Whitton, Leeton and Coleambally, right across to Hay and Moulamein in the western Riverina and some Murray River border towns.

"It's always hard to say how many customers there are that roam on mobile sites, but if you look at the population of those towns [as a start], it's thousands," Mr Taylor said.

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Telstra wasn't yet in a position to put a timeframe on the repairs, as fibre-optic teams were still on-site assessing the situation.

The fix could be a quick one and resolved within a couple of hours or a longer process, depending on the damage, Mr Taylor said.

A spokesperson for the company reiterated that fixed services aren't affected - with the exception of some customers impacted around Maragle and Tooma - and that landlines and NBN should still be operating as normal.

"The trouble is when people are on the road or travelling, or out on a property, that's where we need to get them connected," they said.

"We know how important it is for people to stay connected, and are treating these repairs with our highest priority and we apologise for any inconvenience when unforeseen damages like this takes place."