The acclaimed pianist performed a solo at an MSO event in August, prefaced by a dedication to journalists killed in Gaza.
The orchestra then asked him not to perform at a scheduled concert at the Melbourne Town Hall, before cancelling the entire event over safety concerns.
Jayson Gillham alleges the Melbourne Symphony Orchestra has discriminated against him. (HANDOUT/EMBLEM ARTISTS)
In an open letter, the orchestra's lawyer Leon Zwier said Mr Gillham had made "outrageous demands" for compensation and other relief, which it promised would never be met.
"Your letters make serious allegations about breaches of the law by MSO. They are wrong and Mr Gillham knows they are wrong," Leon Zwier from Arnold Bloch Leibler said.
The pianist had abused his position by using a concert to air his political opinions, the letter stated.
In response, Gillham said he was deeply disappointed by the letter and alleged the orchestra had discriminated against him.
"The implication that my actions caused safety concerns is unfounded and deeply troubling," he said in a statement.
The orchestra's actions infringed his workplace rights and his right to freedom of expression, the pianist said.
They constituted direct discrimination due to political belief or activity, protected under the Equal Opportunity Act, and also violated protections under the Fair Work Act, he claimed.Â
Gillham stated he had initially requested a public apology, compensation for reputational damage, and affirmation of artists' rights to free speech, and future performance opportunities with the MSO.
He had also asked the orchestra to commission a piano concerto by a Palestinian composer and make a donation to a Palestinian music organisation.
As the dispute escalates, the orchestra is already in disarray, with musicians passing a vote of no confidence in senior management, and the recent departure of managing director Sophie Galaise.
Former arts minister and musician Peter Garrett will lead an independent investigation into the incident with consultants KPMG Australia.
The review will examine MSO's policies, procedures and protocols including freedom of speech and artistic expression on stage.
On Friday, Gillham and composer Connor D'Netto released the music that sparked the dispute as a fundraiser for a charity to support Palestine.
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