Immigration law, student visa, employment visa, partner visa
It is not unusual for workers to be asked to perform extra duties within their normal work hours. eg a checkout operator could be required to stack some shelves or to sweep up some spilled goods. However if they were asked to perform at a higher level than their appointed role, provided these extra duties were within their capability, they are required to do them but they are entitled to extra pay, usually called 'higher duties' for this work.
As a 187 visa holder your employment is subject to rules outlined click here
I expect that if you wish to move to another employer who is willing to sponsor you and qualifies under the rules, you can do so.
If you are being exploited you can complain to immigration, but if the employers right to employ you is revoked, and you do not find an alternate employer, you may be required to return home, but I'm not sure of the actual rules. Someone else may be able to reply to this.
Is there a migrant resources center in your area. They may be able to assist or point you to an appropriate body.