Phoenix Natural Gas must make changes to the way it bills customers who pay by direct debit, a watchdog has said.
A consumer watchdog has criticised Phoenix's billing policy
The General Consumer Council said some householders who pay their bills by direct debit have received invoices asking for hundreds of pounds.
The money has to be paid to make up the difference between the amount they pay in monthly instalments and the actual cost of their annual gas bill.
The council said it would like to see Phoenix issue bills more frequently.
Joanne Gamble of the General Consumer Council said more needed to be done by the company, even though it has recently changed its billing policy.
"We have been working very closely with Phoenix over the last year on the frequency of their bills," she said.
"We would like to see a very clear cost and benefit analysis of increasing the number of their bills."
Alliance assembly member Naomi Long, a Phoenix customer who pays by direct debit, said she discovered her bill was underpaid by £513.
She said the company, who had estimated her monthly bill would be £35 per month, demanded the excess amount was paid immediately.
"As a consumer I felt I had been misled from the very outset," she said.
"Obviously people are going to sign up to gas if they know they are going to have low bills.
"They have a responsibility to their consumers to give them proper advice and let them know if those monthly payments are not enough to cover their bills."
Naomi Long, a Phoenix customer, pays by direct debit
Phoenix now sends a bill to debit payers every six months, but only for the first year. Customers can also telephone the company directly for billing updates.
The company also said it now monitors the gas used by new customers more closely to try and prevent underpaying by direct debit.
However, Ms Long said it should not be up to customers to find out if they are underpaying their bills.
"It puts the onus on us, and runs up our phone bill ringing them for information, when every other utility will tell you how much your bill is going to be," she said.
"I think fundamentally that is very unfair on people."