A NSW Supreme Court judge has ruled an internet cartoon in which lookalike child characters from The Simpsons engage in sexual acts is child pornography.

In a landmark finding, Justice Michael Adams today upheld a decision convicting a man of possessing child pornography after the cartoons, depicting characters modelled on Bart, Lisa and Maggie engaging in sex acts, were found on his computer.

The main issue of the case was whether a fictional cartoon character could "depict" a "person" under law.

"If the persons were real, such depictions could never be permitted,"Justice Adams said in his judgement. "Their creation would constitute crimes at the very highest end of the criminal calendar."

Alan John McEwan had been convicted in the Parramatta Local Court of possessing child pornography and of using a carriage service to access child pornography material, the latter of which has a maximum penalty of 10 years' jail.