Here are some options you could use, depending on the context:
General:
* Ma ka hana au. (This translates to "I am working".)
* Aia au ma ka hana. (This means "I am at the work place".)
More specific:
* Ke hana nei au ma [name of workplace]. (This means "I am working at [name of workplace]")
* Aia au ma ka hale hana. (This means "I am at the work house.")
Formal:
* Ke hana nei au i koʻu hana. (This means "I am working at my job.")
The best way to say it depends on the specific situation and who you are talking to.