The Australian prime minister, Tony Abbott, came under fire after he was pictured with his arm around the Duke of Cambridge at an engagement on Thursday.

The photo, uploaded on the Australian Harper's Bazaar Instagram feed, attracted criticism from users, with one describing it as "awkward" and another saying "not appropriate".

Abbott is not the first Australian prime minister to be draw criticism for apparently breaching protocol. Former prime minister Paul Keating was dubbed "the Lizard of Oz" after putting his arm around the Queen in 1992, and his successor, John Howard, was also photographed committing the same offence. He strenuously denied making contact with her.

Michelle Obama also affectionately put her arms around the monarch, in 2009, which caused an uproar on British shores. The Queen did not seem to mind as she reciprocated the hug.

But what is the correct etiquette when it comes to interacting with the royals?

According to the Daily Mail, "protocol dictates that no commoner should touch a royal, except for a handshake if an arm is first extended to them".

Dr Kate Williams, a historian at Royal Holloway, University of London, seems to back this viewpoint, stating in an interview with the BBC that royals "are treated as people set apart from the rest of us", which dates back to medieval times when monarchs were believed to have been divinely appointed to rule by God.

When asked about Abbott's actions, a spokesperson for the royal family confirmed that he did not breach protocol, adding that individuals should observe "normal social etiquette" when meeting the royals.

The royals have taken steps to appear less divine starting with Princess Diana and now with the Duke and Duchess of Cambridge, who seem to have been tasked with modernising the brand.

Some day in the future, a "commoner" will hug a royal, and no one will bat an eyelid.