The former speaker of the House of Representatives visited wineries on 14 occasions in 2010 and left taxpayers to foot the $954 limousine bill.
The first trip involved stops at seven wineries, the second outing involved five and on the third trip he inspected two wineries.
Justice Burns said “it is not possible to give definitive meaning to the term” parliamentary business.
“The ambiguous definition of parliamentary business means that one cannot conclude that the purchase or consumption of alcohol, or meal, by a parliamentarian is inconsistent with a journey being taken for the purpose of conducting parliamentary business.”
Justice Burns said the prosecution had failed to rule out Mr Slipper had travelled to the wineries for “purposes of informing himself about those businesses as part of his function as a parliamentarian”.
He also said it could not be ruled out that Mr Slipper met third parties for the purpose of parliament.
As a former speaker Mr Slipper will keep his generous pension and gold card travel entitlements.
Both the government and the Opposition yesterday said MPs were aware of the high standard the public expects from them in claiming travel expenses.
Whereas so far as the law is concerned there is no standard at all.