The Australian actress, who is of Indian heritage, is the third person in recent weeks to allege they experienced racism while working for the long-running soap.
She left the show in March after four years playing the character Dipi Rebecchi.
In a lengthy statement, Johal, 33, who is based in Melbourne, said she suffered taunts from white castmates, and was made to feel more targeted when she asked for help from senior crew members.
The former lawyer said it was a "human rights issue" and she felt "morally compelled" to speak out.
It comes after two Aboriginal stars - Meyne Wyatt and Shareena Clanton - also claimed racism on set. The latter said it was "traumatising to work in such a culturally unsafe space" when describing her experiences.
Production company Fremantle Media said it was looking into the claims.
In her social media post on Tuesday, Johal said she had experienced "direct, indirect and casual racism" on the set.
She said one person compared her to a bobble-head toy, and repeatedly pretended to be Indian character Apu from The Simpsons in her presence "with accompanying Indian accent and movement of head.”
The star said she asked her castmate to stop but they did not.
Another castmate, who is still on the show, referred to her as "you people", she claims.
She also said the same castmate allegedly called her "the black one" or "blackie" to other members of the crew behind her back.
Johal claims the same actor said Indian actors had been hired to "fill their diversity quotas" and "not because they’re any good."
The actor said when she raised her complaints with management, no disciplinary action was taken. She said her complaints meant she was targeted further.
"While they were sympathetic and the actor [was] questioned on one occasion .. no further action was taken."
Clanton praised Johal’s expression on Tuesday, commenting on her Instagram post: "So it begins... I am with you and so proud of you for speaking up."
In response to Johal’s allegations on Tuesday, Fremantle Media said: "We remain committed to ensuring a respectful and inclusive workplace for all employees on the set of Neighbours and take very seriously any questions about racism or any other form of discrimination."
A 2018 Screen Australia industry study found 7 per cent of characters on screen in the country were from non-European backgrounds, compared to 17 per cent of the population.
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