Go onto the OMARA website and type in the name of an agent to check if they are registered

A bridging visa is granted to allow you to remain in Australia while you are awaiting the processing of an application, or to provide you with lawful status while you make arrangements to depart Australia.

Generally speaking, the main reason that visa applications are refused is due to applicants submitting the application with incomplete information or not responding to requests within the prescribed timeframe. This is where using a Registered Migration Agent can save you time and money by ensuring your application is complete and decision-ready.

There are many other reasons why Australian visa applications get refused including:

1. providing false and incorrect information;
2. missing or insufficient evidence;
3. not providing proof that you are competent in the English language (student and skilled visas)
4. nominating the wrong occupation from the Skilled Occupations List (SOL);
5. not showing incentive to return to home country with a visitor visa application;
6. not meeting the health requirements 

If your visa is refused or cancelled, you might be able to have the decision reviewed by the Administrative Appeals Tribunal (AAT).