Missing payments on loans, credit cards or bills can send a person's rating downwards.
Johnson said the first he knew of the issue was when he received a notification from Credit Simple - an arm of Illion that allows the public to find out their credit score for free and track it.
"I got a notification from Credit Simple telling me that my credit score had been lowered about a month ago and emailed Vodafone asking why."
Before he could get an answer Johnson received a second notification saying another 50 points had been taken off his credit score - a total of 100 points - bringing his score down to around 500.
The 25-year-old says he contacted Vodafone's customer services number but was referred to its credit team which only had an email address and was then told he needed to call customer services.
"I was hot potatoed between teams."
In frustration he emailed chief executive Jason Paris which prompted a response from Vodafone's customer solutions team leader apologising for his experience and promising to investigate it with the credit review team.
Johnson said it appeared he had had his credit rating downgraded due to a late payment on a $16 bill, which had not appeared on the MyVodafone app, which was how he normally pays his account.
But a Vodafone spokeswoman said Johnson had been late paying his bill three times this year, missing due dates in January, May and June.
"This meant his account was flagged with an independent credit rating company, who has then decided to deduct points from his credit score."
She said like any service provider it asked customers to pay their bills on time and payment due dates were clearly marked on emailed invoices and within the MyVodafone app.
"If a customer is late in paying their bills, and hasn't contacted us to seek payment relief, their account will be tagged as having a 'late payment'.
Johnson said Vodafone sent their bills out a month in advance, so paying the bill the following month was "completely regular for Vodafone customers."
And he said, according to the app his balance was showing as zero owed.
"I paid via the app, and the app reported that I had $0 outstanding."
The Vodafone spokeswoman said Johnson's June invoice was emailed out on May 21 which was well within its service agreement.
"The invoice would have also been available on the MyVodafone app at this time. We can see no "glitches" and nothing specific to this customer."
Johnson said he was now worried his low credit rating would affect his ability to borrow money to buy a house in the future.