The Environmental Risk Management Authority says there are not existing rules to stop some bomb ingredients being sold online.
The internet auction site TradeMe says it has cancelled the account of a user who was selling chemicals that can be used in making explosives.
TradeMe says immediate action was taken once it became aware of a user selling chemicals that could be potential bomb ingredients.
However, the same ingredients are still on offer from other vendors.
ERMA says all the chemicals it has seen listed on TradeMe are approved for sale, so there is no obvious legal recourse.
The agency says it is working with TradeMe to look at ways to monitor or control future sales of potentially explosive ingredients.
And it says it would consider reviewing its regulations on the sale of such chemicals if there appeared to be a need.