Ms Gates told BBC News NI that Nipsa represented administrative staff who look after IT in NI Water, and also those who test the water quality.
She said the new salary structure “changed the terms and conditions of workers and it means now that the workers doing the same job are being treated differently”.
“The next stage of the action will depend on how management respond to our requests for negotiations to resolve the issue,” Ms Gates added.
“We stand ready to resolve this dispute, to negotiate our way out, but that will require the management side accepting that we do need to negotiate for workers who have been put on separate terms and conditions.”
Ms Gates said the impact of the strike can depend on the weather.
“The weather is due to take for the worst and it’s likely to be colder so that might change the impact that the strike action has,” she said.