Visitors will be able to head up to the roof until 10pm as the house stays open late for the last two days of the festival, taking in sweeping views towards Old Parliament House and the lake.

Parliament House is open late during the festival and plays host to a number of events, many of which are sold out, such as a special dinner in the Members' Guests Dining Room and an underground tour with a geologist.

There is an interactive family tour on Friday and Saturday which has children finding clues and solving puzzles linked to the Magna Carta, and there will be free screenings of the various Robin Hood movies from 6.30pm - again linked to the Magna Carta which has its 800th anniversary this year.

The Queen's Terrace cafe will also be open late with a cocktail bar and snacks.

The idea behind the free rooftop access is to allow people to enjoy Enlighten from a new perspective, high above the buildings and crowds on the lawns.

Visitors will still have to go through security before they can get into any of the events and extra security staff have been put on to handle the crowds.

Department of Parliamentary Services head Carol Mills said people would need a ticket to get onto parts of the roof but other parts would be free.

She said Parliament House's Enlighten events had proved very popular with visitors.

"Each year we've tried to expand the Enlighten program to put something special in and something new," she said.

"We've been very, very happy with the response this year. Each year we've been trying to grow."

On the first night of Enlighten, the house played host to a sold out "Sunset on the Roof" event where 200 people had drinks and snacks on the rooftop with music.

Ms Mills said there had been a "very, very fast response" to the event and this year's Enlighten program at Parliament was the largest in the three-year history of the festival.

Special behind-the-scenes events, such as a "white glove art tour" of the Parliament House art collection, had even drawn visitors from out of town.

"We've even had people come down from Sydney and interstate specifically for the art tours," she said.

For more events see aph.gov.au/Visit_Parliament/Visitor_Events/Enlighten_2015