Optus has scored the job of deploying free customer Wi-Fi at 21 major Westfield shopping centres around the country, with more to follow.

Apart from providing marketing opportunities and customer intelligence, offering free Wi-Fi encourages people to spend more time in the centre. And centre managers already know that the longer people are under their roof, the more they spend.

Yes, it's arguable which way that works: if you've got a lot of shopping to do, it'll probably take longer. But lingering in the centre provides additional opportunities to tempt you.

Anyway, pilot installations at Westfield Miranda (New South Wales) and Westfield Garden City (Queensland; pictured) found those who used the service stayed 12% longer than those who didn't.

How could they tell? If a phone's Wi-Fi interface is active, it broadcasts its MAC address. So by recording when every device appears and disappears the operator can determine whether those that do actually connect stay longer than those that don't.

Phone vendors can choose to make things a bit more difficult. For example, Apple's iOS 8 introduced a scheme where the device initially broadcasts a random MAC, making tracking harder until a connection is made using the real MAC.

Optus is using Aruba and SkyFii equipment and services for the rollout.

SkyFii collects large amounts of customer data from free Wi-Fi services and provides analytics including behavioural profiling to its clients. (Remember, there ain't no such thing as a free lunch.) The company recently announced a reseller agreement with Mnet Mobile.

"Our focus is on delivering the most compelling ICT solutions to support Australia's leading enterprises and government organisations," said Optus Business managing director John Paitaridis.

"We are delighted with the Westfield partnership as we demonstrate our capability in managed WiFi and analytics solutions on a national scale."

Scentre Group (the company behind the Westfield brand in Australia) CEO, Peter Allen said "we believe that this service will meet the growing shopper demands for data as they increasingly use their smartphones to inform their shopping trip and connect socially."

The other 19 centres on the 2015 rollout schedule are Belconnen (ACT); Bondi Junction,
Burwood, Chatswood, Hornsby, Kotara, Liverpool, Parramatta, Sydney (Myer side), Tuggerah, Warringah (New South Wales), Geelong, Southland (Victoria); Chermside, Helensvale (Queensland), Tea Tree Plaza (South Australia); and Carousel, Innaloo, Westfield, Whitford City (Western Australia).

Existing Wi-Fi services at the Sydney (New South Wales), Fountain Gate (Victoria) and Carindale (Queensland) centres will be upgraded by Optus during 2015.

Additional Westfield centres will be Wi-Fied during 2016.