Matthew Paul Hockley admits he added the teenager on Snapchat and sent her messages, but says he was interested in the girl's mother.

But the 33-year-old's explanation "just doesn't ring true", crown prosecutor Judy Geary told a jury on Friday.

Hockley is accused of grooming the girl while a senior constable working at the Inala Child Protection and Investigation Unit. He has since resigned.

He added the girl on the social media app days after interviewing her over naked photographs she had circulated.

Over three weeks from July 30, 2019 Hockley and the girl exchanged more than 1000 chat messages and 30 images.

In messages photographed by the girl he wrote, "heading home to get naked and relax" and "rather be chillin with you".

In another he said, "I was so cold last night. Needed you to warm me up".

Hockley told the Brisbane District Court the messages were inappropriate, "an overshare" and "a horrible use of language".

"It's stupidity, that's what it is," he said about one message.

When asked whether he was trying to desensitise the girl to the idea of sex, he replied, "Absolutely not".

"The reason why I joined the police was to help people," he said. "My role is to protect children not to harm them."

While he should have kept things professional and at arm's length, he had no sexual interest in the girl, his barrister Matt Black told the jury.

"His poor judgment at the time does make him stupid, (but) it doesn't make him guilty," he added.

Hockley's interest in the teenager's mother helped explain and understand his involvement with the girl, Mr Black said.

Over the weeks he was chatting to the teenager the officer also had a consensual "sexual encounter" with her mother, telling the court his marriage was "broken almost beyond repair" at the time.

The prosecution has argued the former officer invited the teenager on holiday, told her he slept naked and called her "beautiful" in some messages to her. He is also accused of commenting on sexual interactions between older men and girls.

The court heard the girl said some messages from Hockley "were creepy", making her feel uncomfortable.

No messages could be retrieved during the investigation because Snapchat messages - which can include photos or text - are automatically deleted after being viewed, Ms Geary said earlier. Senders are also notified if someone takes a screenshot of conversations.

But the girl did take pictures of three conversations using another device.

Ms Geary said the tone of those messages was relevant because they showed the conversations were not "banter between friends".

"They demonstrate his interest in her, his interest in spending time with her and his willingness to sexualise the chats between them," she told the jury in her closing argument.

Hockley pleaded not guilty on Monday to one count each of grooming a child under 16 with intent to procure engagement in a sexual act, grooming a child under 16 with intent to expose to indecent matter and using the internet to procure a child under 16.

Judge Bernard Porter is expected to sum up the case on Friday afternoon before the jury retires to deliberate.

Australian Associated Press