After having his contract terminated, Winstanley had entered the firm's home page and left an obscene message to the board of the company.
Defence lawyer James Elliot said the event happened after Winstanley "took a number of tablets by way of an overdose" and he did not recall the offending.
Afterwards he apologised and removed the message.
Judge Morris said it was a serious offence and "particularly so when it's a breach of trust, and someone has been given access to do it".
However, the judge said Winstanley's blame was reduced because of a "very rapid guilty plea" and personal factors.
She said she "was concerned a discharge would hide factors an employer should know about" given the offence is classified as dishonesty.
Winstanley had been advised by a potential employer that a conviction would rule out his employment but, if he was successful in gaining a discharge, he could reapply.
Judge Morris said she was satisfied a conviction would bring "consequences out of all proportion to the gravity of offending", which is the legal test for a discharge, and she ordered payment of $1500 in reparation.