Not in Australia:
From a March 2003 Sydney Morning Herald
article:
... the Child Support (Assessment) Act allows courts to order repayments where a presumed parent is later found not to be the biological parent
So you may have to pay the support now, but if you later find out that you were not the father, you can claim it all back, as happened last year:
In a precedent last year in Victoria, Liam Magill successfully sued his ex-wife for $70,000 in damages and economic loss after DNA testing proved she had deceived him by telling him her lovers' children were his own.