The owner of Adelaide's second-largest shopping centre has blocked off a section of its car park used by commuters and hospital workers following state government legislation proposed to make instituting paid parking more difficult.

Early this morning, Westfield cut off parking at Tea Tree Plaza near the Modbury Hospital and the O-Bahn bus interchange in a move it said would allow more space for shoppers during the day.

The car park on Smart Road was reopened at 8:45am — before most shops open, but after the peak demand from commuters and hospital workers.

Westfield wants to introduce paid parking at Tea Tree Plaza, prompting a Labor election promise and legislation now before parliament to force shopping centre owners to get council approval before making motorists pay for spaces.

Centre management has previously claimed about a third of spaces were taken up by people who were not shoppers.

In an email sent to shoppers on its mailing list yesterday, Tea Tree Plaza management said the move to ban parking in the southern car park before 8:45am would "address the availability of parking bays" at the centre.

"In response, from Tuesday 18 October, our car park adjacent to Modbury Hospital will open at 8.45am.

"As you know, we also have a broader proposal to introduce managed parking at Westfield Tea Tree Plaza. It is designed to prioritise your parking needs. We are planning to include three hours of free parking as part of the change.

"The SA government has introduced legislation into parliament which, if passed, will prevent us from making some of these positive changes.

"If we're unable to introduce managed parking, we may consider further adjustments to the parking arrangements at Westfield Tea Tree Plaza to ensure your parking needs, as customers, are prioritised."

Lachlan Monfries, who is the regional manager at Scentre Group (which owns Westfield), said the other car parks were open as usual. 

"Providing an easy and accessible parking experience for customers when they visit is our priority," he said in a statement. 

"Customer feedback has supported the need to change how the car park operates in order to prioritise the availability of parking bays for customers."

Union slams move

The state's retail workers union has hit out at the move by Westfield to restrict parking access.

Shop, Distributive and Allied Employees' Association state secretary Josh Peak said it put the safety of workers at risk. 

He said this morning's restrictions caught out workers, who had to park elsewhere.

"All we've been saying is 'work with us, work with the community, let's find a solution that doesn't see workers have to park an incredibly long way from work and risk their health and safety'," he said.

He said Westfield was trying to get around the need for council approval for parking restrictions.

"What we think is going on here is they're saying, well we're going to start putting in parking controls because we can't get our way with the installation of boom gates and look it's really very strange behaviour, very aggressive behaviour and we think completely unnecessary," he said.

Premier Peter Malinauskas said Westfield's move showed it had other options to control parking apart from making people pay for it.

"I think that's just a demonstration of the fact that Westfield has options to be able to manage their parking systems without installing boom gates, which of course is their plan, a plan supported by [Opposition Leader] David Speirs and the Liberal Party that I am fundamentally opposed to," he said.

The opposition opposes the government's legislation, which was introduced last month.

Opposition planning spokeswoman Michelle Lensink said the proposed laws sent an "appalling" message to businesses wanting to invest in South Australia.

"We're aware of other shopping centre sites where they're keen on investing hundreds of millions of dollars. That's on hold because the Malinauskas Labor government approach to managed parking just sends a terrible message to anybody who wants to do business in South Australia," she said.

Work is set to start on a new $43.5 million Park 'n' Ride car park at Tea Tree Plaza later this year.