By common law I mean court-made law. The judges in the higher courts give reasons for their decisions, and it is this long-winded process of reasoning that forms the law. There is no way of knowing it than by reading them. It is highly convoluted because each judge gives their own take on the entire case, and they may follow different lines of reasoning as to different issues. Then you need a squad of QCs to argue for a week over what they all meant. In the end the Court just makes it up as it goes along. Great stuff LOL.

Finding the cases is bit of an art in itself, which is why a special caste of law librarians have been bred up since the Middle Ages, complete with cowls, cassocks, and fusty looks.

austlii has the High Court judgments, for example:
http://www.austlii.edu.au/au/cases/cth/HCA/

The leading case, where the High Court first considered the correct interpretation of the Convention, was Chan's case:
Chan Yee Kin v Minister for Immigration & Ethnic Affairs (1989) 169 CLR 379 at para. 11
http://www.austlii.edu.au/au/cases/cth/HCA/1989/62.html

This is foundational. It set down the standard of proof, which means you can't find it in the Convention - you have to look Chan's case.

The Australian common law of refugee status then comprises all the decided cases of the Federal, Full Federal and High Courts, of which there are zillions. You can see lots of refugee cases here:
http://www.austlii.edu.au/au/cases/cth/HCA/toc-M.html
because the Minister of Immigration has been named as the plaintiff, so it's alphabetically under M.
But of course there's all the other ones where the refugee was the plaintiff, so there's all the Chans, the Wu Shan Liang's, and so on.

Then the Migration Act was amended so refugees names are anonymized, so you get cases with names like SZGJV. It's great.

Interesting you should mention habeus corpus because this is one of a related group of court orders known as the prerogative writs, to what are called “inferior jurisdictions” – meaning either lower courts or executive officers - to prohibit them from doing something unlawful, e.g. deporting someone; or to compel them to do something the law requires - e.g. hearing an application according to law.

The prerogative writs are a creation of the common law, which means they are not created by any statute, but yet they control and limit the exercise of executive power in interpreting and carrying out all statutes including the Constitution.

“ If they haven't crossed the Border officially then I can't see why they can't be refused a visa”

If they haven’t officially entered Australia, then you have a valid point. However lots of them have. Remember there’s not just the ones coming by boat. Most onshore applicants come by plane.

“I can't see how, "removed from Australia to a receiving country, there is a real risk that the applicant will suffer significant harm" would cause significant harm provided the receiving Country wasn't the one they fled from”

An example would be the Malaysia Solution case:

In that case, the Gillard gumment amended the Migration Act to empower the Minister to declare that another country was safe for Australia to remove asylum-seekers to. The Minister declared Malaysia.

But the High Court said that decision was not lawful, because Malaysia had not signed the Convention, so there was nothing stopping Malaysia from returning the asylum-seekers to their home country. Hence in sending them to Malaysia, Australia was in breach of its undertaking in the Convention, not to return a refugee to the risk of persecution. (PNG and Nauru have signed the Convention, obviously.)

You should read it: Plaintiff M70/2011 v Minister for Immigration and Citizenship. It will give you an idea how mind-numbingly complex and impenetrably long-winded the cases are.

Another doozie was where the High Court decided the Immigration Minister can’t outsource a refugee’s character assessment to ASIO. Advanced hair-splitting; very well remunerated