* Dollar index near 15-mth low, eyes 75 support
* Euro resilient despite S&P's downgrade of Portugal
* Australian dollar dips, but near 29-year high vs dollar
* Dollar/yen steady, talk of offers above Y81.50
By Naomi Tajitsu and Masayuki Kitano
HONG KONG/SINGAPORE, March 25 (Reuters) - The dollar hovered near a 15-month low against a basket of currencies and was in sight of a 29-year trough against the Australian dollar as a bounce in equities suggested that risk appetite was on the mend.
The euro dipped briefly after Standard & Poor's downgraded Portugal's credit ratings by two notches to BBB and warned it could cut it again by one notch as early as next week. [ID:nL3E7EO3OJ]
But the single currency's losses were limited, and the broader picture was one of dollar weakness on the back of a recovery in global equities over the past week and improving risk sentiment.
"The market is treating many of these downgrades as rear-guard action which are already well discounted and the dollar is under pressure broadly," said Todd Elmer, currency strategist at Citi in Singapore.
"This will continue to support the euro even if we do see some marginal negative news on the sovereign debt crisis," he said, adding that he saw the euro retesting its recent highs in the near term.
The euro has shown resilience despite worries over the fiscal woes of highly-indebted euro zone countries such as Portugal, supported in part by market expectations that the European Central Bank would raise interest rates as early as April, boosting its yield advantage over the dollar.
The euro held steady from late U.S. trade on Thursday at
$1.4174
There was talk of bids in the euro around $1.4140 to $1.4150, while euro offers were cited near $1.4210 to $1.4220.
The single currency is not far from a 4-½ month high of $1.4249 hit earlier this week. If that level is breached, the next significant peak on charts is the early November high of $1.4283.
In another sign of the dollar's broad weakness, the Australian dollar hovered near its Dec. 31 peak around $1.0257 -- a level not seen since 1982. The Aussie dollar last stood at $1.0189, down 0.2 percent from late U.S. trade on Thursday.
"With U.S. equities rising on solid earnings it looks as if risk appetite improving," said a trader for a major Japanese bank in Tokyo.
A plunge in Japanese equities last week on worries over the economic toll from the earthquake and tsunami that struck Japan's northeast on March 11 and fears over a quake-crippled nuclear power plant had spurred a bout of risk aversion.
But markets have since settled down, especially after the Group of Seven industrialised nation's joint intervention to stabilise the yen last Friday, the trader said.
The dollar index, which measures the dollar's value against a basket of currencies, rose about 0.1 percent to 75.699 <.DXY>, hovering near a low of 75.340 hit earlier this week, its lowest since early December 2009.
The dollar index has looked vulnerable on charts after its recent drop below long-term trendline support connecting its July 2008 low and November 2010 trough, and one possible downside target is its 2009 low of 74.170.
A trader at another Japanese bank in Tokyo said the 75 level is now seen as major support for the dollar index. A breach of that level could open way for further fall in dollar, he added.
The dollar held steady against the yen at 80.99 yen
Market players are bracing for the possibility of further yen-selling intervention by Japanese authorities if the dollar drops below 80.00 yen. But dollar selling interest by Japanese exporters ahead of the month-end is seen likely to temper any gains in the dollar.
The trader for a major Japanese bank in Tokyo said there were dollar offers at levels above 81.50 yen. (Additional reporting by Hideyuki Sano in Tokyo; Editing by Kim Coghill)