The NSW Fair Trading Commissioner will investigate whether the organisers of Sydney's Gay and Lesbian Mardi Gras breached Australian Consumer Law by misleading people who purchased tickets to the official after-party.

The party, which kicked off after Saturday's Mardi Gras parade and ran until 8am on Sunday, was headlined by international acts Sam Smith, Dua Lipa and Kesha.

Dua Lipa, centre, was one of the headline acts at the Mardi Gras after-party. Credit:AAP

Mardi Gras organisers confirmed 10,000 people purchased tickets to the sold-out event at the Entertainment Quarter at Moore Park. But almost half of those partygoers missed out on the highlight performances of the night with the main venue, the Hordern Pavilion, quickly reaching its capacity of 5500. Dozens of ticketholders, who were stuck in queues for hours, took to social media on the night to criticise the event's organisation.

Tickets to the after-party included access to four smaller stages in the precinct, where lesser-known artists performed.