They ran off and asked a couple walking their dog to call police.
Mr Alexander hoped his team would be able to get the girls' full account in the next few days.
Officers who dealt with them immediately after the assault were provided with a detailed description.
Mr Alexander said the man was Indian and about 1.8m in height and skinny. He was in his late 20s or early 30s with dark skin, short black spiky hair and a moustache.
But the key to identifying him, said Mr Alexander, was his blue Superman T-shirt and black-and-white checked shorts.
"We're appealing to anyone who was in the park at the time to contact us."
Police were yesterday conducting a scene examination and had interviewed four witnesses.
"We want members of the community who use the park to be vigilant," Mr Alexander said. "It is important that people do not walk alone."
Anyone with information should contact Detective Sergeant Albie Alexander on (09) 261-1300, or Crimestoppers on 0800-555-111.