A bit of a confession Friday with two of our largest companies revealing lower earnings and sales guidance, and both blaming the high value of the Australian dollar for much of the pain.

Telstra was first with its update at 8.35am and Foster’s followed 30 minutes later. The abacuses must have been working overtime last night at the Melbourne HQs of both companies.

Telstra’s update immediately raised the question why Telstra is fighting the federal government (a willing buyer) for a bunch of assets from which customers are walking away.

That’s a question that has also escaped the federal Opposition, especially Nick Minchin, who led the parliamentary opposition to the plan. It has also escaped a bunch of mostly Sydney-based institutions.

Telstra blamed the dollar, tough competition in Hong Kong and the fact that customers were abandoning its fixed wire business. So why is it fighting so hard for the old-fashioned copper wire phone lines and associated business, and why hasn’t it dumped them all in Stephen Conroy’s eager lap? For a suitable price, of course. But it’s a slowly dying business.

Telstra’s update revealed a downgrading of sales to basically flat for the 2010 year and “negative” for the December 31 half. That’s down from low single-digit growth (1%-3% maybe).

Telstra blamed the strength of the Australian dollar, “tough operating conditions in Hong Kong,   strong domestic competition driven by ULL growth and very competitive mobile offers; an accelerated move to wireless-only homes, which is impacting revenue in PSTN and fixed broadband products.”

And that’s the argument against Telstra hanging on to the copper wire. The billions of dollars of assets in the old switched network are being competed into the ground, especially as people go for what’s called “naked DSL” where they get broadband at home without the phone service, or just get a mobile phone, no broadband, and no home phone.

There are competitors offering (and Telstra as well) wireless broadband (3/Vodafone, etc), which are giving people 3G data and phone links and leaving the Telstra copper wire in the dark.

And then there was Foster’s. Its update didn’t directly mention the impact on group sales and profits (in percentage terms, such as Telstra did), but the strong Aussie dollar is carving millions off the earnings of the company’s stuttering wine business and that forced Foster’s to point out that many analysts had unrealistically low currency values in their forecasts.

“Unfavourable exchange rate movements are expected to negatively impact first-half wine earnings by between $80 million to $90 million,” the company explained.

“The major currency impacts are expected to be an approximate nine cent increase in the average US dollar and an eight pence increase in the average pound sterling exchange rates compared to the prior period.”

Foster’s said exchange rate movements had a transaction and translation impact on earnings.

“Foster’s notes a wide range of currency assumptions in current broker analyst forecasts, some of which are out of step with prevailing rates.”

That’s as good as saying earnings will be down, without having to put a figure on it, just in case it’s a bit wrong, even though there’s only 14 days to the end of the first half and 2009.

There’s also strong discounting of wine in the US, wine sales in Australia and New Zealand are being affected by falls in volumes in commercial wines partially offset by growth in premium wines.

But at least the old standby, beer, is chugging along, without a forex to worry it:

“Carlton & United Breweries (CUB) continues to perform in line with Foster’s expectations,” the company said. “Beer drinkers of Australia save Foster’s from wine hangover” might be the headline when the company reports first half earnings on February 16.

More companies are going to be revealing the bite of the dollar in their interim updates over the next month.