Before a packed Brisbane courtroom, the 43-year-old former prestige real estate agent elected to step into the witness box after the prosecution closed the Crown case against him. Dressed in a dark suit, glasses and a yellow polka-dot tie, Mr Baden-Clay took an oath on a bible at 2.50pm.

Mr Baden-Clay is accused of killing Allison Baden-Clay at their home in Brookfield on April 19, 2012, and dumping her body. He has pleaded not guilty to one count of murder.

He took a sip of water before being questioned by his defence counsel, Michael Byrne, QC.

''Did you kill Allison?'' Mr Byrne asked.

''No, I did not,'' Mr Baden-Clay replied.

Mr Baden-Clay denied fighting with his wife on the evening of April 19, or the morning of April 20. Her also denied leaving his children home alone while he disposed of his wife's body in Kholo Creek at Anstead.

He broke down in tears a he spoke of his love for his wife of 15 years.

''I fell in love with her,'' he said.

''I fell in love with her pretty much straight away … I knew that she was the one.''

He went on to speak about his relationship with his wife, recalling the day he proposed to her under a replica Eiffel Tower erected above a Park Road cafe in Milton.

Mr Baden-Clay rambled at times during his testimony, which lasted for 1½ hours, explaining the sequence of events which led to each milestone in their lives, including a trip to New Zealand when he recalled swimmer Kieren Perkins won Olympic gold.

He also listed his academic and business achievements and his roles in community organisations, including the Brookfield State School P&C Association and the Kenmore Chamber of Commerce.

However, he spent much of his time in the witness box chronicling his wife's battle with depression.

Mr Baden-Clay said his wife started showing signs of depression during their trip to South America in 1998, which he attributed to the controversial anti-malaria drug Lariam the pair had been prescribed.

''It had no effect on me. It obviously had a dramatic effect on Allison,'' he said.

Mr Baden-Clay said his wife withdrew socially and developed severe mood swings, which he could not understand.

Her symptoms worsened when they travelled to the Swiss Alps to volunteer at the Kandersteg International Scout Centre, he said.

Mr Baden-Clay is the great-grandson of the founder of the scout movement, Lord Robert Baden-Powell.

''When we first arrived … there was a tremendous snowfall right across the alps that winter,'' Mr Baden-Clay said.

He said Mrs Baden-Clay did not respond well to the threat of avalanches, becoming ''very, very anxious''. ''There were times when she thought, in her paranoia, that the sky was falling,'' Mr Baden-Clay said.