The 3-year-old was found in a council sewage pond just over a kilometre from his home in January 2019.
Two police investigations found Lachlan accidentally drowned but his father, Paul Jones, said that conclusion lacked common sense.
Paul has given evidence to the inquest into Lachlan's death over the past two days.
Coroner Alexander Ho earlier heard accusations from Paul's lawyer Max Simpkins that Lachlan's mother, Michelle Officer, and her two older sons killed the boy, before storing his body in a freezer and then dumping it into the sewage pond.
Those allegations led to a clash between Robin Bates - the lawyer for police, Paul, and the coroner.
"I don't know what's happened on this day. I don't know how my son got out there. I'm here to find the answers," Jones said, in response to Bates' questioning.
"I have never said that Michelle [killed Lachlan]. There was [Michelle's two older sons] Cameron, Johnny there and Michelle. I want to know what happened."
Numerous witnesses had told the initial investigators or the coroner they had seen Lachlan or someone potentially fitting his description running towards the sewage ponds on the night he went missing.
Bates asked Paul if those witnesses were wrong, but Coroner Ho intervened.
"The veracity or otherwise of the evidence which has been given is, again, a matter for me to determine, not Mr Jones," Ho said.
But Paul did answer Bates' line of questioning.
"Are you claiming they're the people that you want to have seen Lachie - if you had done your job properly, we wouldn't be here today," Jones said.
The father did not believe police had offered himself or Lachlan the respect or effort he deserved.
"It just seems like a bloody botched up police job and you're trying to fix it," he said.
Paul had met Michelle Officer more than a decade ago.
Paul told the coroner they were living together within a month of meeting.
He was over the moon when he found out Officer was pregnant with Lachlan, he said.
"I was more than excited because I was told I wasn't able to have a child, so I couldn't believe it," he said.
"Michelle had been pregnant before and we lost that one."
Her two older sons to another father were good with Lachlan when he was first born, Paul said.
"They were all right at the start but things deteriorated later. I probably didn't do with those two boys had previously done when Lachie was born because Lachie was my own and I focused on him."
He said his relationship with the boys deteriorated with Cameron moving in with his father, followed by Jonathan, though he later moved back in with Officer and Paul.
In August 2018, Paul assaulted Officer - he told the coroner he had pushed her - and their relationship ended as a result.
"I take full responsibility for it," he said.
"I had an issue where I was brought before the courts and Michelle gave a statement in here that it was male assaults female, it wasn't, it was just a push, it was a common assault. I have on numerous occasions roared at those boys, especially for the way they were treating Lachlan. I take full responsibility for that. I did get very hot tempered."
He had little access to Lachlan in the period which followed, but two days before the boy's death he stayed at Officer's home, sleeping in the lounge with Lachlan.
Paul also stayed on the night of 28 January.
He and Lachlan woke early the next day.
"I think it was just before six and Lachie woke up and put his arms around and sort of squeezed my neck and said 'Dad, I don't want to live here anymore, can we go to a motel'," Paul told the coroner.
Officer made him eggs for breakfast and Paul kissed her and Lachlan before going to work. It was the last time he saw his son alive.
Paul did not believe Lachlan could have walked the 1200 metres to the sewage pond.
He described his son as a "lazy little bastard".
He also disputed others' evidence that Lachlan was energetic, prone to running away and a good climber.
Paul said Lachlan's behaviour and attitude changed in the months before his death.
"He was more agitated and anxious - he just wasn't the same kid," Paul said.
He had concerns about how Officer's sons had treated Lachlan, particularly an incident where Cameron had told him he was a naughty boy and smacked his hand, and another occasion where Jonathan brought a friend around who was drunk and had urinated on Lachlan.
"I just have grave concerns - the way Lachie was treated in the last eight months - that something sinister has happened. I'm not saying who has done it. But I'm saying my son didn't walk out there."
