I got the job!! so when do i tell them i am pregnant?

I am 16 weeks pregnant, working a job that i absolutely despise. I have nightmares about my boss sacking me. Its just really crap. So ive been applying for work and omg I got a job. Its just cashier and nightfill at the local grocery shop but its gonna be a lot easier on me then what ive been doing which has been very physical work. I didnt say I was pregnant at the interview because I knew they wouldn't hire me if I was honest and upfront about it. I am employed on a casual basis and expected to start next Wednesday. By law in Australia, I dont have to tell an employer I am pregnant until 28 weeks but I am starting to show.. Should i tell them ASAP or wait a few weeks to prove i am a valuable employee THEN tell them? Again, I am a CASUAL employee.