Tim Marney, one of the State's most senior public servants, is leaving his post as Under- Treasurer after nine years and will take on a senior role at WA's Mental Health Commission.
Mr Marney, who earns more than $500,000 a year, is leaving 3½ years into a five-year contract renewed by the Barnett Government in June 2010.
His deputy, Michael Barnes, is fancied to step up into the role.
Mr Marney has famously attracted the ire of Colin Barnett over the years, including in 2009 when he told journalists he was yet to be convinced the Government's policy of retaining Royal Perth Hospital as a tertiary facility was affordable.
Appointed to the role as a 35-year-old by the Gallop government in 2005, Mr Marney earlier served his apprenticeship as deputy under treasurer to John Langoulant.
In 2008, Mr Marney predicted the Nationals' power-sharing demand that 25 per cent of mining royalties be spent in the regions would blow the State's budget and threaten its triple-A credit rating, which was downgraded in September.
Government sources yesterday were at pains to emphasise Mr Marney was not pushed, but conceded mounting pressure from the Premier's office may have been a factor in his departure.
Mr Marney has been on the board of depression charity beyondblue since 2008 and has publicly spoken of being diagnosed with chronic anxiety disorder in 2003 and his battle with depression.
His departure from Treasury adds to a list of major public entities the Barnett Government needs to find bosses for.
It is still looking for a Health Department director-general more than a year after Kim Snowball announced his retirement, and in November Corruption and Crime Commissioner Roger Macknay quit two years into a five-year term. Mr Marney said he could not comment.