The secretary of the Department of the Prime Minister and Cabinet, Martin Parkinson, is to retire, as expected. His career swan song, however, was anything but expected.

When Tony Abbott became prime minister in 2013, one of his first acts was to sack three department heads for reasons that were, by all accounts, ideological and superficial. Don Russell was dismissed for his Labor links, Blair Comley because of his portfolio (he led the now-abolished Climate Change Department) and Andrew Metcalfe (Immigration) for reasons that remain a public mystery.

Dr Parkinson, an excellent secretary who had also led the Climate Change Department, had moved to the Treasury by then. He was to have been Mr Abbott's fourth victim; Mr Abbott said the Treasury boss had agreed to step aside after spending a few months helping the new government establish itself.

However, Dr Parkinson proved indispensable and was retained. When Malcolm Turnbull later rolled Mr Abbott to become prime minister, he appointed Dr Parkinson to the bureaucracy's top job. This journey says everything one needs to know about Dr Parkinson's ability and professionalism.

If one had to criticise any aspect of his leadership, one could argue he was too responsive to the government of the day, to the point where he openly undermined the Freedom of Information Act - a law he was obliged to uphold - pushing the specious argument that it hampered public servants' ability to give frank and fearless advice. If anything, the act can temper the quality of advice by exposing it to public scrutiny. Nonetheless, Dr Parkinson deserves to be remembered for his better qualities.

The more surprising announcement on Thursday was the decision to replace Dr Parkinson with the present Treasury secretary, long-time Coalition adviser Phil Gaetjens. The news provoked an immediate response from opposition parties.

The Greens' spokesman on public-sector issues, Adam Bandt, labelled the appointment "a direct attack" on the bureaucracy.

Labor, via its finance spokeswoman, Katy Gallagher, was more measured. She warned the government against politicising the public service, adding: "We're being quite fair in saying 'let's see how Mr Gaetjens goes'."

This is a significant improvement in the party's attitude since the election campaign earlier this year. Labor's shadow treasurer, Chris Bowen, said he would not have confidence in a Treasury led by Mr Gaetjens, and that, if the ALP won office, he would not accept an incoming brief from Mr Gaetjens.

These personal attacks on Mr Gaetjens were unwarranted. While he had served as an adviser to Liberal ministers for a decade, Mr Gaetjens is essentially a career public servant. He began at the Bureau of Transport Economics in the 1970s and spent most of his working life in the federal, South Australian and NSW bureaucracies.

Top public servants often make a point of working in ministers' offices as personal advisers, to broaden their experience rather than push an ideological agenda.

By way of example, Department of Home Affairs chief Mike Pezzullo divides opinion like few other public servants. Many regard his speeches and other public forays as partisan advocacy on behalf of the Coalition. Yet his only direct political experience is with the Labor Party; first with foreign minister Gareth Evans and, later, with opposition leader Kim Beasley, trying to kick the Coalition out of office.

In recent years, the public service has become a political battleground. Some Coalition politicians tend not to trust it and, as a result, attack it reflexively. Labor, in opposition at least, seeks to cast itself as the bureaucracy's champion, making promises it won't necessarily keep in office.

None of this rhetoric helps public servants. They shouldn't be seen through a partisan lens; they take pride in avoiding bias and should be trusted to continue to do that. Politicians serve the bureaucracy best when they leave it out of political debate entirely.