Airports have reduced car-parking prices and improved services but are still monopolies that need regulation, competition watchdog chief Rod Sims says after revealing profits at the four biggest airports rose 6 per cent in 2017-18.

"I'm very pleased with the improvements in this report, but how much of that is due to the Productivity Commission review and how much these gains will be sustained we just don't know," Mr Sims told The Australian Financial Review.