In the vaults of Archives New Zealand lies a unique collection of several thousand fading letters, photographs and papers. The Nash Collection offers a window into a world gone by.

Former prime minister Sir Walter Nash became involved in local politics from his arrival in Wellington in 1909. His personal papers are a treasure trove of information about World War II, the birth of the New Zealand Labour Party, as well as every noteworthy issue of the day.

Without them, a hole would exist in the nation's historical record.

From the same building, chief archivist Marilyn Little will soon start an investigation into the deletion of Prime Minister John Key's text messages.

Her inquiry stems from a request by the Green Party. It is a spot of political point-scoring, exploiting Key's embarrassing friendship with hit-job blogger Cameron Slater. But politicking aside, the investigation is truly important.

In the age of the spin doctor, we now rarely know what a politician really thinks. Their response to a crisis is packaged up into palatable soundbites for news bulletins. Biographies, sympathetically penned by acolytes and admirers, have become another election campaign weapon.

But years later, after the politician's death, papers, communications and diaries are archived for the historians, researchers and academics to pore over. The most valuable lay bare the private thoughts of a national leader - revealing more than we could ever hope to guess in real time. These are the foundations of text books and gripping historical biographies.

It would be a shame if Key's correspondence was forever confined to the digital dustbin. He has been in the front seat during some momentous events in New Zealand's history - the Canterbury earthquakes, Pike River mining disaster, historic Treaty settlements, two royal visits and ground-breaking diplomatic relations with China.

He "textses" regularly with other world leaders - and has talked publicly about receiving messages from British Prime Minister David Cameron, Prince William, Aussie counterparts Julia Gillard and Tony Abbott.

Key deletes his text messages for security reasons, should his phone fall into the wrong hands. Given the recent scrutiny over his textual relations with Slater - and the recent rash of malicious hacking - he has every reason to be wary.

No doubt, he also wants to keep some parts of his already very public life to himself. He might well text wife Bronagh - "QT Gr8, home at 7pm, put the pasta on", or "where r u Max?" to his teenage son. The country does not need to see those messages.

But many of his other SMSs - those sent as prime minister - will one day be as important to the historical record as the letters of Winston Churchill.

There are already clear guidelines in legislation covering official information to distinguish what should be considered private and public. Little should be able to offer reassurance to both Key, his successors and the public that national leaders can shield their personal lives without resorting to destroying a priceless historic record.