Some states have issued guidance for this situation, indicating you do not need to file or pay state for a temporary stay. Hawaii, so far, is not one of them. Reference: https://www.hodgsonruss.com/assets/htmldocuments/Telecommuting_5.22.20.pdf
Technically, you are required to file a HI return, but "nobody" does. See http://money.cnn.com/2013/06/28/pf/taxes/business-traveler-tax-threat/
The general rule is: your report all your income on your home state return, even the income earned out of state. You file a non-resident state return for the state you worked in and pay tax to that state. Your home state will give you a credit, or partial credit, for what you paid the non-resident state. You will have to file a non resident HI state return and pay HI tax on the income earned there.. You will also file a CA full year resident return and calculate tax on ALL your income. CA will give you a credit, or partial credit, for the tax you pay HI. So, there will be little or no double taxation, but you have the cost and hassle of filing two state returns. Do the nonresident state return first.
