I am an Ethiopian, I am flying to Sydney, AU via Beijing. Can I visit Beijing ?

Of course you can visit Beijing. If your layover in Beijing is not more than 24 hours, you don't need to apply for the transit visa and you are allowed to leave the airport by applying for a temporary stay permit when passing through the immigration counter. If your layover is longer, you need to apply for a G type entry permit in advance so that you can tour the city.