Premier Kathleen Wynne says she is being transparent about cap-and-trade costs for natural gas customers because they know what the average impact will be.

The Liberal government's cap-and-trade plan is expected to add about $5 a month to home heating costs starting Jan. 1, but those increases will be buried in the "delivery" line on natural gas bills.

The Ontario Energy Board announced earlier this year that cap and trade won't be a separate line item because the regulator considers them a cost of doing business.

The auditor general found that 89 per cent of natural gas ratepayers want the cap-and-trade costs clearly displayed, but the regulator has so far stood firm, and the energy minister said he won't intervene.

Wynne says her government has been clear about the average increase to natural gas bills, so people are aware of that.

She says the average increase has been talked about for months, so her government has "been very transparent."