In the past, any time, that Australia's borders have appeared at all weak, this has been exploited by people smugglers in south east Asia. Significant numbers (thousands) of people have tried to enter Australia by boat during these periods, hundreds have died trying to do so. This is dangerous to both Australia's immigration security, and to the people these people these people smugglers exploit.
The only effective way Australia has found to stop this happening is to have extremely strict border laws founded on the statement by Prime Minister John Howard in 2001 that "we will decide who comes here, and the circumstances under which they arrive."
We have basically stopped illegal immigration by that route into Australia when we put in place, strict, inviolable rules.
Anything that makes those laws look like they've been weakened is likely to lead to a resumption of those boat arrivals.
So we don't make exceptions. We don't weaken them.
This graph is a little out of date now - in 2013, the border laws were tightened again after the Labor government that was elected in 2007 weakened them and we're back down to basically zero numbers - but it does illustrate the difference we see when our laws are very strict, and when they are seen as relaxed at all - because the law change in 2007 certainly wasn't open borders - it was still pretty strict. But pretty strict isn't enough. It needs to be rock solid.
There's other factors too - complexities relating to High Court decisions - that I can into if you really want me to. But the danger here is greater than I think a lot of people easily understand. A lot of it is about international perception as to how strict our rules are - the people smugglers will exploit anything to convince people they now have a chance to get in, that the rules have been weakened at all.