Every Sunday in the centre of Jakarta, tens of thousands of Indonesians flock to what is one of the most congested roads in the city.

From early to mid-morning, there is an opportunity to breathe freely on what is known as "car-free Sunday".

As the name suggests, cars are banned and the road is opened to a sea of humanity in a city where wide open spaces are few and far between.

This particular Sunday, a group of women see an opportunity to preach a stricter form of Islam.

The group's leader, Triken Sholihat, fears Indonesian Muslim women are becoming too liberal in what they wear.

"We will help them to wear the head scarf properly while also explaining the reason and importance of wearing the hijab according to sharia," she tells the ABC.

"Many women are not aware that wearing the hijab is obligatory according to the Koran."

Ms Sholihat and her team have brought with them boxes of coloured veils which they distribute for free to women whose head coverings are either non-existent or, in Ms Sholihat's view, too liberally worn.

"We are campaigning for women to wear the head scarf properly, though wearing face cover is not compulsory, it is an option," she says.

She says the hijab should never be transparent and should be long enough to cover a woman's chest.

She says some Indonesian women are covering their hair but not their upper body, which is attractive to men.

Some women very willing to cover up more

With the veils in hand the women approach those passing by to give the garments away and teach the recipients "the best way to wear them".

Intan Permatedewi jumps at the chance for a free veil and lesson in wearing it.

"I did try and wear a hijab before, but some parts are still exposed, while this one is according to sharia and is fully closed," she says.

"From now on I will be wearing the hijab in line with sharia guide."

Ismawati, 12, is also approached and agrees to a dressing lesson.

"I only wear a hijab at school. They asked me if I'd be willing to wear one now and I said I will," she told the ABC.

An older woman, Nining, is also happy to oblige.

"My hijab was not covered enough like this one, a long cover. With God's will, I will correct the way I wear the hijab," she says.

Shift to Arab interpretation of Islam in Indonesia

Indonesia is widely seen as a moderate Muslim nation, but there is concern that some religious teachings are changing and that beyond the hijab, women are being urged to also cover their faces.

The head of Indonesia's largest Muslim organisation, Nahdlatul Ulama, which has more than 40 million members, says there is a growing trend that is seeing a shift to an Arab interpretation of Islam, which in part is also leading to more Indonesian women wearing full face-covering veils.

One woman teaches another about the correct way to wear a head scarf. ( ABC News )

"The indication is that there has been a very radical change, not only the change in thinking but the way one is presenting themself in fashion," Nahdlatul Ulama chairman Aqil Siradj says.

And while there are no official figures, he believes there are noticeably more Indonesian women choosing to fully cover their faces.

"I learnt Islam from all parts of Indonesia and none of the Ulemas ordered me to wear Arabic long dress, none of them ordered their female students to wear face covers," he says.

Amanda, 32, started wearing niqab, a veil that covers the entire body and face apart from the eyes, after she graduated from university a decade ago.

"Wearing a face cover is about the strength of faith in somebody, if someone lacks the faith they won't be able to wear a face cover," she says.

"People are wearing face covers because of their obedience to God, what I see from year to year are more women covering their faces."

'When I took it off they were shocked'

But while many Indonesian women choose to wear headscarfs and increasing numbers are donning full face covers, there are also those who are deciding to take them off.

Nur Masuroh removed her hijab a decade ago.

"At the time it was hard because I used to wear a large head scarf, and I was amongst friends who were involved in Islamic activities and when I took it off they were shocked," she says.

But she says in contrast to her decision, more women in her circle of friends are now wearing full face cover.

"Lately, I've seen that many people, friends and others wear it [face cover]. They started wearing small head scarves and then gradually wore larger ones and in the end cover their face," Ms Masuroh says.

"We can see them in train stations, public places. Back in the early 2000s there were only a couple of them around and now there are many more."

But for now there is only anecdotal evidence of a shift in Islamic teachings and dress and while Indonesia is the world's largest Muslim nation, it is also the world's third-largest democracy, meaning most Indonesian women should be able to wear whatever they like.