Queen Elizabeth turned 90 on her actual birthday, which is April 21, and a wide range of events has been taking place in the UK for the longest-reigning British monarch.

Queen Elizabeth has been on the throne for 64 years but in keeping with tradition to ensure celebrations take place in the (northern) summer, she also has an official birthday in June.

To mark the occasion, the sixth and final portrait in the series of official birthday pictures taken by US photographer Annie Leibovitz has been released, showing the monarch with her husband at Windsor Castle, SBS News reported.

At the weekend, the Queen and her family attended the traditional Trooping the Colour parade of soldiers in ceremonial uniforms in central London followed by a fly-past of Royal Air Force aircraft over Buckingham Palace.

To conclude the celebrations, the Queen hosts The Patrons Lunch for 10,000 guests at the largest street party to be held on the Mall, the avenue that leads to Buckingham Palace, an event organised by her grandson, Peter Phillips.

Guests came from some of the more than 600 charities and other organisations of which she is patron with the festivities beamed to large screens in the capital's parks.

In Port Moresby, the Queen's Birthday will be celebrated at the Grand Papua Hotel's Executive Lounge from 12 noon to 2pm in the form of an afternoon tea.