"Harry never stops talking about marriage and children, and she has now got used to the idea. The wedding is likely to take place next year."
The prince, 29, and Bonas, 24, were introduced by his cousin, Princess Eugenie, in May 2012. But she felt too young to marry.
"Cressida has been in floods of tears," one of her friends claimed at the time. "Harry is coming on very strong, but she is all over the place. She thinks she is far too young to get married and is scared by all his talk of marriage and settling down."
But the prince persisted. When he was sent to serve in Afghanistan in November, the pair were in regular telephone contact throughout his four-month posting. Upon his return, the romance blossomed.
Bonas studied dance at Leeds University and is studying at Trinity Laban Conservatoire. Her mother is Lady Mary-Gaye Curzon, 66, a daughter of the 6th Earl Howe. Her father is Lady Mary-Gaye's third husband, businessman Jeffrey Bonas.
One of her half-sisters, Pandora, is an accessories designer for Vivienne Westwood; her half-brother Jacobi Anstruther-Gough-Calthorpe is a polo-player. Her sister Isabella recently married Richard Branson's son Sam. Before going to Leeds, Bonas spent a year working in a bar and restaurant in Australia.
A visit to Sydney harbour to inspect a warship parade caused roars of applause wherever he was sighted. "Go, The Ranga," cried some, using the slang term for redheads. Others shouted "We love you Harry" and "Harry, Harry, Harry!"
Georgia Marks, 11, was overwhelmed to have spoken to the Prince, bursting into joyful tears: "He's so nice."
Police were forced to remove Victoria McRae, 20, who ventured on to the harbour on a kayak with a flag that read "I love U Harry". "He's an eligible bachelor, I thought I was in with a chance," she said.
Prime Minister Tony Abbott's daughters Bridget, 20, and Frances, 22, were also keen to meet the prince. An excited Bridget tweeted a photograph of her and her sister with the Prince, with the caption: "A once in a lifetime experience meeting Prince Harry."
Prince Harry's movements in Perth were kept a closely guarded secret.
However, it is believed his short stop was to include a meeting with SAS troops based at the Campbell Barracks in the coastal suburb of Swanbourne.
Before his departure from Sydney, Prince Harry told reporters Australia already felt like a second home and he was very sad to be leaving. The Prince was next scheduled to attend a charity ball in Dubai.
- additional reporting Telegraph Group Ltd