Firebreak shopping rules are causing "uncertainty, fear and anxiety", according to supermarket workers.
Filco director Matthew Hunt said he has had no information from the government and "found out on Facebook".
And one supermarket worker said staff are facing "abuse and intimidation".
The Welsh Government will publish revised guidance later over its ban on the sale of non-essential items in supermarkets during the 17-day lockdown.
Mr Hunt told Radio Wales Breakfast: "We never had any communication from any of the governmental bodies before this was introduced.
"As yet, we've still yet to have anything concrete that we can work with."
He added he was worried about the pressure staff members may feel about deciding what is essential.
"This is only a two-week period, by the time you try and unravel the confusion that is out there, we'll be out the end of this one," he said.
A supermarket worker called Jane told Radio Wales she felt anxious going into work.
"You don't know what you're going to face," she said.
"It is causing a lot of frustration with customers. A couple have behaved really awfully - we had one gentleman come in with no mask, filming the store, shouting abuse, telling us we were all robots for the government.
"We get general frustration from other customers who wanted to buy something for the home and can't. A lot of people tend to mutter, trying to provoke a reaction."
She added she had been "personally intimidated by a customer who's really got in my face and invaded my personal space" and some customers had been removed from the store.
"It certainly isn't right that shop workers should have to put up with that kind of behaviour.
"We've been going to work all the way through the pandemic to keep the country fed and abuse shouldn't be part of our job at all."
Health Minister Vaughan Gething told Monday's coronavirus press briefing that supermarkets should use their discretion over the sale of non-essential items.
Jane, from Mold in Flintshire, said her store had needed to use flexibility when a customer was in crisis at the weekend.
"They needed baby clothing - they'd fled from a bad situation and they weren't able to bring everything that they needed for their baby, so we allowed them to purchase what they needed," she said.
"There was no way you could have said no to that person."
But she added the decision should not be made by supermarket staff.
"It shouldn't be down to the responsibility of the shop worker to enforce the new arrangements.
"It should be up to the public to follow the rules...we need a clear message of what we should say to them."