Keflavík International Airport: Rent a car at the airport and drive via Reykjavík and Route 1: roughly 1 hr 45 min–2 hr to Hella in normal conditions. This is the actually useful option, particularly if Hella is your base for the South Coast or highlands; rental rates are highly seasonal, but expect Iceland-level prices rather than a bargain. Without a car, take an airport coach to Reykjavík, then a southbound Strætó service toward Hella; it is slow, infrequent and requires careful timetable checking, so do not plan a tight arrival-day itinerary around it.
Reykjavík Domestic Airport: For domestic arrivals, collect a rental car and take Route 1 southeast: about 1 hr 30 min. There is no rail network and no meaningful station transfer to Hella; buses can work only for travelers prepared to plan around sparse schedules.