First, as prelude to my answer, I will say that a good answer to your question regarding your ex is beyond the ability of anyone here; if you want a good answer that fits your particular case, then you should definitely consult with a competent local tax attorney to discuss all of the facts and details. Certainly you should not be putting any more details up online and in public if criminal issues are at stake.
Second, as a general matter, your question should really be broken down into two questions:
(1) can a person (who is not a US citizen) who owes a substantial amount of back taxes re-enter the country, and
(2) can a person who owes a substantial amount of back taxes be arrested, either at the time they re-enter the country or otherwise.
As to the first question, the mere fact that someone owes a substantial amount of back taxes to the US government does not mean that they are legally barred from re-entering the country, so the direct answer to that specific question is "no."
As to the second question, as a theoretical matter, yes, someone who owes a substantial amount of taxes can be arrested; however, the number of people who are arrested for criminal tax fraud/evasion is not very large. Basically, very few people ever get criminally prosecuted for tax fraud/evasion; the government would much rather go after them to collect the taxes - plus penalties and interest - rather than trying to prosecute them, if for no other reason than that a lot of people who end up on juries tend to be sympathetic to people arrested for not paying taxes.
In terms of whether a person who owes a substantial amount of taxes - and who otherwise falls into that class of people whom the government wants to arrest for criminal tax fraud/evasion - would be arrested immediately upon entering the country, that most likely depends on whether or not a warrant has been issued for that person's arrest prior to the time that person attempts to re-enter the country. In general, when you re-enter the country the immigration people don't run a check on your passport number to see if you've paid all of your taxes, so merely owing taxes is not going to get you arrested at the immigration counter in the international airport.
So, as a hypothetical answer based on very little concrete facts, it would be reasonable to suppose that unless an arrest warrant has been issued against your ex for criminal tax fraud/evasion, he is unlikely to be arrested at the immigration desk at the airport when he arrives back in the US. However, once he's back in the US he could be arrested if - and I stress "if" - he meets all of the other criteria the government would consider before deciding to issue an arrest warrant for criminal tax fraud/evasion.
It really does come down to whether or not the government is investigating him for criminal tax fraud/evasion, how close they are to having a case against him, and how interested they are in criminally prosecuting him.
It should be mentioned, however, that if your ex is really in trouble for criminal tax fraud/evasion, returning to Australia may not protect him from being prosecuted in the US because the US could request that Australia extradite him to the US if the charges were serious enough.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dwatchley@newyorktaxcounsel or visit my website at www.newyorktaxcounsel.com
Tax obligations are generally civil and that would not prevent your ex from returning to the US. If he does not believe this, you should have him talk to a tax attorney. Your kids certainly have the right and need to see their father!
I hope this helps!
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He'll only be arrested if there's an arrest warrant issued of him - that would require he'd committed a crime (or been charged with a crime and was to appear in court, etc.). Failure to pay taxes is not generally considered a crime and there would be no arrest warrant involved (unless there's a whole lot more than that going on). On the surface, it seems highly unlikely that there would be an arrest warrant issued for failure to pay taxes.
Hope that helps.
Evan A. Nielsen is licensed to practice law in California and handles federal tax matters throughout the U.S. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.