Jeremy Colman was first appointed to the auditor general role in 2005
South Wales Police are investigating the former Auditor General for Wales, at the request of his employers, it has been confirmed.
Jeremy Colman, 61, head of the public spending watchdog the Wales Audit Office (WAO) since 2005, resigned with immediate effect on Wednesday.
It is understood there was an internal review after pornographic material was discovered on his work laptop computer.
The WAO confirmed the resignation followed a personal conduct inquiry.
It is understood Mr Colman initially said he was leaving on health grounds.
South Wales Police said it was investigating an allegation regarding computer material but it would be "inappropriate to comment further at this stage".
The WAO said: "Following an internal review at the Wales Audit Office, matters regarding Jeremy Colman's personal conduct were referred to the police.
"We cannot comment any further but are cooperating fully with South Wales Police enquiries."
Mr Colman had been due to stay in the £170,000-a-year post until 2012 but left on Wednesday without giving a reason.
He was originally appointed for five years in 2005, but that term was extended in May 2009 for a further three years.
Cambridge-educated Mr Colman had a career in the civil service, including the Treasury, before jobs in the private sector in banking and accountancy from the late 1980s. He was then appointed director of the National Audit Office in 1993.
Since the process of selecting a permanent replacement at the WAO will take several months, the assembly is likely to make an interim appointment to the post.
The WAO is responsible for ensuring value for money for more than £20bn of public funds, but an independent review last October criticised top managers.
An external review into the WAO last autumn found serious flaws in management, but it became clear on Thursday that Mr Colman's departure was not linked to this.
The WAO confirmed personal conduct had been the subject of the internal investigation involving Mr Colman.
The International Peer Review, published in October 2009, found unhealthy tensions between senior managers, perceptions of nepotism in decisions over allocation of staff to projects, and significant issues of trust and industrial relations.
The Assembly's Public Accounts Committee (PAC) has responsibility for overseeing the selection process for the auditor general position.
Its chair Jonathan Morgan AM said on Wednesday he had been aware for "a few days" that the auditor general was considering his position .