It's a regular scene outside op shops – people swing by after hours to drop off their donated goods at the front door, or leave them beside an overflowing donation bin on the side of the road.

While those responsible might have good intentions, they're actually breaking council rules.

Illegal dumping was a big issue around the country and a fineable offence.

"From a Council perspective, illegal dumping is an issue that plagues most, if not all local and regional councils," Gavin Pooley, Hamilton City Council's regulatory compliance team leader Waters said.

Gareth Marshall, The Salvation Army's national family store manager, said it was grateful to all who donated, but preferred those donations to be made during opening hours.

"We appreciate that our hours do not suit everyone. There is a very small percentage of people that may abuse this but our focus is on the generous majority."

On Saturday morning, donated goods that had been left outside a Wellington op shop overnight had been sprawled across the footpath. The mess would have most likely stayed that way until staff arrived to collect it when they opened.

"All goods donated are processed by our staff and volunteer for items that can be sold in our stores," Marshall said.

Pooley said illegal dumping on private property, such as the doorstep of op shops, took "time, energy and resources to deal with". 

We appreciate that our hours do not suit everyone. There is a very small percentage of people that may abuse this but our focus is on the generous majority.

"[This] inadvertently takes away time, energy and resources that could be focused on something positive for the community - like the work many of our community organisations (like op shops) undertake."

It also took money away from the stores' vital funding for community programs, as some used the dumped donations as a free shop.

Fines vary between regional councils, but often range from $100 to $400. However, catching those responsible was an issue in and of itself.

It was hard to track down offenders, Pooley said, as it required a lot of time and resources. In some cases, investigators would sift through dumped items, searching for something that could be traced back to that person - an addressed envelope, or clothes with name tags, sometimes CCTV footage would be reviewed to try and locate the registration of the vehicle that dropped off the items.

If offenders weren't identifiable, the cost of removing the items, if they weren't already collected by the stores, fell to the ratepayers.

In Wellington alone, the cost of cleaning up illegal dumping across the city, not just outside of op shops, totalled around $220,000 a year.

Marshall said the majority of its donors had "good common sense", but for those unsure of how to donate he recommended visiting during opening hours or arranging collection with a nearby store.

"If Op Shops have signs requesting goods to be dropped off, they are then responsible for anything that gets left on that site. If they don't have a sign, then it is considered illegal dumping," a Wellington Council spokesperson clarified.