John Simpson, 52, on Monday pleaded guilty to his third charge of trafficking heroin in recent years, with a magistrate telling him his day had come.

"You must have known the risk if you continued to sell heroin," magistrate Michelle Hodgson said while jailing Simpson.

Ballarat Magistrates Court heard Simpson was caught with heroin, including balloons full of it down his underwear, about 2.30pm on January 28 while he was parked in a car in Willow Grove, Wendouree West.

Police prosecutor Leading Senior Constable Pepe Brown said police initially saw bags of heroin under Simpson's feet before he admitted to having more throughout the car.

He later told police during an interview that he had been to Melbourne earlier in the day to collect 3.5 grams of heroin, which he paid $800 for.

The court heard he used some of the heroin himself and sold the rest in 0.1 gram packages for $50.

David Tamanika, for Simpson, said his client had been a long time heroin user following a motorcycle crash in which he hurt his back.

Mr Tamanika said Simpson, however, was not a big time drug trafficker or "go to" man, adding he sold the drug to recoup some of the costs of his usage.

The lawyer asked Ms Hodgson to consider assessing Simpson for a community corrections order and said Simpson was currently attending drug counselling and not using heroin.

Ms Hodgson said there was no alternative but jail.

She said she needed to send a message to both Simpson and the community that drug trafficking would not be tolerated.

Simpson was jailed for four months.

Ms Hodgson said Simpson would have been jailed for eight months if not for his plea.