Welcome! Not sure about Maui but here's from the HPD website. I think they have an office you can register at on Maui but couldn't find it on their website.
http://honolulupd.org/information/index.php?page=gunmain FIREARMS REGISTRATION
You must be 21 years or older to register a firearm. For in state registration, take your firearm in person to the Main Police Station Firearms Unit no later than 5 calendar days from a purchase or acquisition for registration. You need the following when you register your firearm:
Valid photo identification
Your original firearms permit
Name, address, phone number of seller, in state registration only
The firearm for inspection (In a gun case, unloaded)
If you were born outside the United States, bring proof of citizenship such as an Original US Passport, Original Naturalization Certificate, Original Born Abroad Certificate, or if you are in the military your Enlisted (ERB) or Officer (ORB) Record Brief will suffice only if the document states you are a United States Citizen.
Out of state registrants need to take firearm(s), in person, to the Main Police Station Firearms Unit no later than 5 calendar days from arrival. Out of state registrants are encouraged to arrive at the Firearms Unit no later than 3:00 pm. The out of state registration process can be lengthy. If there is a line, we may not be able to accommodate you. There is no 14-day waiting period for Out-of-State Registration. You need the following when you register your firearm:
Rapback fee $42.00. Cash / exact change or Credit Card.
https://ag.hawaii.gov/wp-content/uploads/2013/01/Rap-Back-Facts-Sheet-Final-12-7-16-002.pdfValid photo identification
The firearm for inspection (In a gun case, unloaded)
If you were born outside the United States, bring proof of citizenship such as an Original US Passport, Original Naturalization Certificate, Original Born Abroad Certificate, or if you are in the military your Enlisted (ERB) or Officer (ORB) Record Brief will suffice only if the document states you are a United States Citizen.
Effective February 1, 2016
You may now submit an Online Firearms Account Form through our Online Report System by clicking here.
The following forms are available on this website and may be filled in prior to coming to the Firearms Section.
Firearms Information Form
Medical Information Waiver
Mental Health Waiver
Firearms Application Questionnaire
Married couples who would like to have their firearms jointly registered in both names need to also bring the Original Marriage Certificate.
Reference:
HRS 134-3 Registration, mandatory, exceptions
http://www.capitol.hawaii.gov/hrscurrent/Vol03_Ch0121-0200D/HRS0134/HRS_0134-0003.htm