Vodafone is facing mounting pressure over the potentially unlawful access of a journalist’s phone records, as more details of a leaked email emerged that alleged a manager in the company instructed investigators to “use any means available” to track down the journalist’s source.

The company has admitted that investigators accessed Fairfax journalist Natalie O’Brien’s call and text records in 2011 after she broke a story about a serious data breach within Vodafone.

After it initially denied any improper behaviour, the company referred the allegations to the Australian federal police for investigation on Tuesday.

The company has maintained that Vodafone management did not ask the two investigators who were involved to access O’Brien’s phone records, and that a review commissioned by the company found management had not instructed them to take that action.

The leaked email was reported by the Australian on Wednesday following reports on the weekend. It was written by a Vodafone Australia manager to a global manager in London in 2012 and relays a conversation with one of the investigators in which the investigator alleges he was instructed by another manager to take certain measures to determine the source of O’Brien’s story.

It says the investigator asked “if they really meant ‘any means available including call-charge records and text messages’, to which [the manager] said yes, he did not care how they did this,” the email said, according to the Australian.

It continues, saying that the investigator “asked if this included the reporter as well … [the manager] said yes.”

Vodafone has denied that the investigators were instructed to access O’Brien’s phone. A spokeswoman for the company said it commissioned KPMG to conduct a review into the circumstances around O’Brien’s phone being accessed in 2012, but the review found that Vodafone management had not instructed the two investigators involved.

The company has refused to release the report publicly. A spokeswoman said KPMG found “that there was no evidence VHA management instructed … [the investigators] to review customers’ text messages or that VHA management was aware they had been accessed.”

Greens senator Scott Ludlam has urged the AFP to take up an investigation into the circumstances of the case. The Media, Entertainment and Arts Alliance has described the breach as “unforgivable” and has written to the company asking it to make public documents concerning the breach.