A Sydney teen who snuck 394 MDMA pills into a western Sydney music festival at which a man died says she did so to help improve her family's financial circumstances.

Tina Phan, 18, plead guilty on Thursday at Burwood Local Court to internally secreting hundreds of pills to sneak into the Knockout Games of Destiny Festival in Homebush in December.

Tina Phan (middle) arrives at Burwood Local Court on Thursday. Credit:Kate Geraghty

The state Oztag player, who has just completed her HSC, "felt obligated to try and help out" with her family's financial situation following the end of her parents' 21-year marriage, her solicitor Cheryl Khurana said.

It was an "extremely immature" decision, and Phan is "extremely remorseful and ashamed," she said, adding that a conviction would impact Phan's life and job prospects for years to come.