A household reference person is not provided in the HILDA datasets. Researchers will have different definitions they may wish to apply to define a household reference person. It may depend on their particular research topic or on how they want this definition to apply over time as circumstances within the household change (e.g., if relative incomes levels differ over time, if relationships change over time, or if when someone moves out or in, etc.). Some variables that you might find useful in defining a household reference person is relationship in household (_hhrih), income (_tifefp and _tifefn), owner (_hsoid1 to _hsoid18, but these are only available in some years) and age (_hgage).
Please note that the person numbers (_hhpno) indicate which row on the Household Form that person is listed. The order in the first wave is simply the order the respondent mentions the people in the household to the interviewer. In later waves, joiners are added and leavers are removed and people are shuffled up for the next wave.