An investigation into the police response to the Roast Busters scandal will be ready within two to three months, the Independent Police Conduct Authority says.

The authority was asked by Police Minister Anne Tolley to look into the police's handling of complaints against two Auckland teenagers who boasted on the internet about having sex with drunk and underage girls.

IPCA head Sir David Carruthers said that the investigation, which began in November, had been complicated by the police's ongoing investigation into the alleged crimes.

"We're going well... but it's going to take a little bit longer than we thought it would." Sir David told a select committee at Parliament this morning.

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"We're now getting to a stage where we'll bring all these conclusions to a head - two or three months, probably."

Mrs Tolley asked for an investigation after a 15 year-old revealed she had laid an official complaint in 2011, which contradicted public statements by police.