Dr van Onselen quit his position in March and agreed not to disparage the network or its US-based owner Paramount.
Three months later, the company sued Dr van Onselen after he wrote a scathing column in The Australian, questioning Paramount's plummeting share price and referring to the network as "the minnow of Australian commercial television".
His high-profile lawyer, Sue Chrysanthou SC, argued the non-disparagement clause was overbearing.
She said the commentator would technically be in contempt of court if he was dissatisfied with his Paramount+ streaming service and wrote an email of complaint to the company.
"It puts him in breach if he says to his mates at the pub 'I'm surprised Network Ten purchased that program, it's not a good program'," Ms Chrysanthou told the NSW Supreme Court on Thursday.
"It is a lifetime order being sought against a person whose profession it is to talk, and only being able to speak about Network Ten for his entire life in glowing terms would affect his legitimacy and professionalism as a commentator and as an academic."
Justice David Hammerschlag, who is presiding over the hearing, rejected her assertion.
"Well then why did he sign it? This is a contract case," he said.
Paramount human resources executive Anthony McDonald also gave evidence at the hearing, saying he could not recall a conversation he had during Dr van Onselen's redundancy negotiations.
Ms Chrysanthou suggested Dr van Onselen asked Mr Mcdonald: "If the CEO f***s a goat and everyone was piling on Ten surely I can pile on too?"
The lawyer suggested Mr McDonald responded: "Of course mate, that sounds fair."
Mr McDonald told the court he did not remember the conversation and said it had not happened.
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