On Wednesday, March 7, there was the heading ''Carr should be driving'' and the letter (by Edward Corbitt of Farrer) consists of one sentence which reads: ''How soon until the Labor caucus and Bob Carr realise he would be a better PM than Julia Gillard?''

That jogged my memory so I looked back over the 12 articles of mine published in The Canberra Times last year. I alighted on the piece published on November 18. It was titled ''Nothing to stop a non-MP being prime minister''. The editorial description of the article was, ''It's happened in Greece and Italy and it could happen here.''

When I wrote that article I had no inkling that former NSW premier Bob Carr would soon become a senator. The article dealt with Greece and Italy where technocrats had become prime minister without ever being elected to parliament.

I then raised the question: could this happen in Australia? To that I answered, ''In practice it is very unlikely. However, it should be noted that there is nothing in our Constitution to stop it.''

I now wish to withdraw the sentence ''In practice it is very unlikely'' and replace it with this: ''With the arrival of Bob Carr in the Senate it is distinctly possible.''

Permit me to quote my next paragraph: ''The office of prime minister is nowhere recognised in the Australian Constitution. Technically she or he is nothing more than any old minister. The relevant provision can be found in the third paragraph of section 64 … ''

I then quoted the Constitution's words which are: ''After the first general election no Minister of State shall hold office for a longer period than three months unless he is or becomes a senator or a member of the House of Representatives.''

I then asserted: ''It is possible to be a minister in the Australian federal government without having been elected by the people in any sense.''

The example I quoted was the case of Queensland senator Santo Santoro who was the minister for health and ageing in the Howard government. I can now add another case: Bob Carr. In accordance with the provisions of section 15 of the Constitution Carr is now a senator, having been ''chosen by the Houses of Parliament'' of New South Wales.

I trust readers will not object if I now quote at some length from my article of November 18 last year.

''The question then becomes: can a senator be prime minister? Must he not resign his Senate seat and follow John Gorton who never met the parliament as prime minister and a senator?

''To know the answer to this question I turn to India which has a number of constitutional features similar to Australia.

''Back in May, 2009, India's Prime Minister, Manmohan Singh, received much praise for his victory in that country's 15th general election.

''Among his several distinctions was that he became India's first non-Hindu PM in a country which is 85 per cent Hindu. He is a Sikh. This was widely noted by international commentators.

''However, in my opinion (and of much greater relevance to Australia) there was another, more important, distinction: he won a general election at which he was not a candidate.

''Singh became prime minister unexpectedly in 2004. After Sonia Gandhi led her Congress Party to victory she turned down the chance to be PM. The job went to Singh.

''India's upper house is called the Rajya Sabha. It is like our Senate in that it represents the states. However, it is elected by the politicians of each state, not the people.

''Manmohan Singh has represented Assam in the Rajya Sabha since 1991, when he became finance minister. When he became prime minister he did not ask that a by-election be created for him in the lower house (Lok Sabha) because there was no need for him to transfer. Instead, a special seat was assigned for him when he needed to attend the lower house. That is rather like the Australian practice whereby the treasurer of a state (if an upper house member) would present his budget in the lower house.

''In Tasmania upper house ministers actually attend question time in the lower house to answer questions on their own portfolios .

''Why did Singh not contest a seat in the Lok Sabha at the 15th general election? A simple answer: he chose not to. There is no requirement that an Indian prime minister must be a member of the Lok Sabha.

''In Australia there is no reason why there cannot be a senator prime minister who chooses to stay in the Senate.''

Essentially I am saying that, perhaps in April next year, the Labor caucus decides Julia Gillard cannot win the general election likely to be held in October 2013.

So Senator Robert John Carr becomes Labor leader and prime minister. Peter Garrett, having announced his intention to retire but not wishing to resign immediately, offers his seat to Carr. Consequently Carr becomes the Labor candidate for Kingsford-Smith in respect of the general election - but there is no by-election. Meanwhile Carr sits in the Senate but goes to the House of Representatives to answer questions at question time. That would require a resolution of the House to allow someone not a member to be present purely for that purpose - but also to make major statements in the lower house when thought appropriate.

All the above may seem strange but I can assure readers that it is perfectly correct constitutionally.

The big difference between Singh and Carr, therefore, would be that Carr would have sought a lower house seat at the appropriate (that is, the next) general election whereas Singh never saw the need to do that.

Malcolm Mackerras is Visiting Fellow in the Public Policy Institute, Australian Catholic University, Canberra Campus. malcolm.mackerras@acu.edu.au