Gerard Baden-Clay has entered the witness box at his trial in Brisbane to deny killing his wife Allison and disposing of her body.

Former real estate agent Gerard Baden-Clay has gone under oath to deny murdering his wife, following his surprise decision to give evidence at his trial.

Under questioning by his barrister the 43-year-old denied murdering Allison Baden-Clay and disposing of her body.

“Did you kill Allison?” Michael Byrne QC asked shortly after his client entered the witness box in the Brisbane Supreme Court.

“No I did not,” replied Baden-Clay.

“Did you fight with her on the evening of the 19th [April 2012]?” Mr Byrne asked.

“No I did not,” he replied.

“Did you at that time leave your children alone in the house to go to Kholo Creek bridge [where his wife’s body was found]?”

“Definitely not. Never.”

Asked about injuries on his face the morning he reported Allison missing, he replied: “I got them when I was shaving that morning.”

Led by Mr Byrne, the father of three gave a detailed account of his early marriage as his relatives and those of Allison watched on.

He remained composed through most of his 90-minute testimony but his voice cracked when describing how they began dating in 1995.

“I fell in love with her pretty well straight away,” he said through tears.

“I had had a couple of girlfriends previously but I felt a level of emotional attachment to Allison that was far deeper than I’d ever experienced before.”

Baden-Clay said they married in 1997 before travelling through Europe, the Middle East and South America.

It was in South America that his wife began acting strangely after taking the antimalarial drug Lariam.

“Her mood fluctuated from what I would call `normal Al’, who is just a lovely, kind, gentle person but with a great sense of humour and a love of life through to some really deep depression,” he said.

Things didn’t improve with the birth of their first daughter in 2001 and Allison suffered severe panic attacks.

“To be perfectly candid with you it came to a point where I was doing 80 to 90 to 95 to nearly 100 per cent…I was doing everything.”

Allison miscarried in 2002, then fell pregnant again in 2003, when Baden-Clay said she saw a psychiatrist and was prescribed the anti-depressant Zoloft.

She then had fewer episodes of anxiety, he said.

But as his real estate business boomed his love life was non-existent and he formed a physical relationship with staff member Toni McHugh, the court was told.

“Gerard doesn’t shrink from this, he won’t say he was ever proud of it,” Mr Byrne said.

“After the relationship and the problems that had been present with Allison, he found himself being flattered, being appreciated and acknowledged.”

Mr Byrne said his client “enjoyed the physical relationship” with Ms McHugh but never wanted to leave his wife and children.

The court was adjourned until Monday when the accused is expected to resume giving evidence.

The defence will also call a forensic psychiatrist and a forensic toxicologist, and show time-lapse video of the flow in Kholo Creek to support its case.

Allison Baden-Clay’s body was found on the banks of the creek at Anstead in Brisbane’s west on April 30, 2012.

The discovery was made 10 days after her husband reported her missing from their home in nearby Brookfield.