Introduced to help make the national science and technology centre accessible to all, the free weekend is expected to attract thousands of visitors across both days.

Guests to the centre will be greeted by a giant, animatronic, anatomically correct spider – benignly named Cherub – in the entrance foyer as part of the latest exhibition.

As well as fake spiders, real creepy crawlies are on show, including some of Australia's most dangerous.

Visitor operations manager Tristan Hoffmeister said the Spiders exhibition, produced in conjunction with the Australian Museum, was one of their most popular.

He said visitor entry had increased, up 7 per cent to date on last year's figures, despite the introduction of paid parking in the parliamentary triangle in late 2014.

Mr Hoffmeister said the paid parking scheme had actually benefited visitors to Questacon, who no longer had difficulty in finding a park.

That might change this weekend though, as young and old flock to the discovery centre.

"We really want to provide science opportunities to all, regardless of their background, abilities or their means to pay the admission," Mr Hoffmeister said.

This comes as member for Canberra Gai Brodtmann said that under a Labor government, admission to Questacon would be reduced by $7.50 per person.

In a joint statement with opposition minister assisting the leader for science, Senator Kim Carr, Ms Brodtmann said while the attraction saw ever-growing visitor numbers, "families under pressure from tight budgets are missing out".

Mr Hoffmeister encouraged everyone who hadn't been to Questacon "since their year 6 excursion" to join in over the weekend and rediscover the attraction.

The Spiders exhibition is on until October. Mr Hoffmeister said while it was still a secret, the next exhibition would feature aeroplanes.

"If you like aeroplanes, you'll like it," he promised.

"I think more people are probably afraid of spiders than they are of aeroplanes, but we can help debunk people's flight phobias as well."