* S&P cuts outlook on AAA rating to negative from stable
* Says "material" weakening in govt debt position expected
* Economists don't expect rating action before Oct 6 budget (Updates with comments from Treasurer, fund manager)
By Swati Pandey
SYDNEY, April 8 (Reuters) - Global ratings agency S&P on Wednesday lowered its outlook on Australia's coveted 'AAA' rating to "negative" from "stable" in anticipation of a "material" weakening in the government's debt position as it splashes out a large fiscal stimulus package.
S&P affirmed Australia's prized rating but said a downgrade was possible within the next two years if the economic damage from the COVID-19 outbreak is more severe or prolonged than it currently expects.
Australia is among a handful of countries in the world to boast the best ranking from all three major ratings agencies.
But it has come under a cloud as the pandemic has dealt Australia a severe economic and fiscal shock, with S&P predicting the A$2 trillion ($1.23 trillion) economy would plunge into recession for the first time in nearly 30 years.
This would cause a "substantial deterioration of the government's fiscal headroom at the 'AAA' rating level," S&P said in a statement.
Treasurer Josh Frydenberg said the outlook downgrade was "a reminder of the importance of maintaining our commitment to medium term fiscal sustainability."
The government has pledged A$320 billion ($197.73 billion) in fiscal spending, or 16.4% of annual economic output, to backstop the economy and prevent a crisis as the pandemic shuts companies and leaves many unemployed. fund managers said Wednesday's outlook downgrade was unlikely to raise the government's borrowing costs by much though it could hurt Australian companies whose ratings are dependent on the sovereign rating.
"A large proportion of credit funds are mandated to maintain funds in a specific ratings bucket," said Asmita Kulkarni, Director Investment Strategy at FIIG.
"With potential widespread downgrades we could see funds being forced to sell-down investment which would result in a widening of credit spreads."
Australian long-dated bonds sold off after S&P's outlook downgrade with 10-year yields AU10YT=RR jumping to 0.967% from 0.909% at Tuesday's close.
Economists said they do not expect a rating downgrade prior to the federal budget due on Oct. 6.
It was only in September 2018 that S&P upgraded Australia's outlook to "stable" from "negative" as the budget came close to balance. The government had even projected a surplus for the current fiscal year and next.
While all those predictions are now under water, Australia's public debt is still in good shape, S&P noted.
"While fiscal stimulus measures will soften the blow presented by the COVID-19 outbreak and weigh heavily on public finances in the immediate future, they won't structurally weaken Australia's fiscal position," S&P said.
"This expected improvement is a key supporting factor of our 'AAA' rating."
($1 = 1.6297 Australian dollars) (Editing by Shri Navaratnam and Jacqueline Wong)