Tasmania Police First Class Constable Jarrod Lightfoot gave evidence in Matthew James Badkin's Launceston Supreme Court trial on Wednesday.

Mr Badkin, who police believe was one of the last people to see Christopher Dean Watkins before he disappeared, has pleaded not guilty to perverting the course of justice in relation to the missing person and murder investigations.

Mr Watkins was last seen at a Box Street unit at Mayfield on August 7, 2013.

Police video interviewed Mr Badkin three times and he completed a statutory declaration regarding his whereabouts on the night Mr Watkins went missing. The interviews were played and the statutory declaration was read to the jury.

Mr Badkin was confused about what he did, who he was with and where he went after he dropped Mr Watkins off near Newnham IGA, the court heard.

First Class Constable Lightfoot said false information in an investigation could take police down avenues that were a waste of time.

Mr Badkin initially told police he was smoking marijuana with Christopher Brewer-Parker at his girlfriend's house, but in another interview said he might've gone hunting at Deloraine.

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While giving evidence on Wednesday, Cassandra Simpson told the jury she called and messaged her partner, Mr Badkin, multiple times in the early hours of August 8 because she was going to a Pink concert in Melbourne and Mr Badkin was going to care for their two children while she was away. The court heard Mr Badkin didn't bring friends over to the house because Ms Simpson didn't want them around her children.

In his closing address, Crown Prosecutor Luke Brett said the accused lied to police to cheat the system.

Mr Badkin told police about people he saw and places he went earlier in the night, but Mr Brett said the accused conveniently had no memory of the crucial part of the night.

Defence lawyer Charmaine Gibson said her client constantly told police he was confused about where he was that night.

"Whatever police are asking him, he is answering," Ms Gibson said.

"This is not a man who thought up a story and is sticking to it."

Mr Badkin's told police he had no reason to remember the night Mr Watkins allegedly disappeared.

Mr Brett rejected the claim, saying the accused first interview was six days after the disappearance.

"How often does a person you know go missing without a trace?" Mr Brett said.

The jury will continue deliberating on Thursday.

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