By Soumya Bhamidipati of RNZ

As Alexandra residents wake to another cold morning, a silent figure slips into the mist. They leave no trace - except for another bottle of flavoured milk dumped at a local petrol station.

Members of the small South Island community have noticed a strange recurring phenomenon. Every few days, flavoured milk is left on top of a particular pump at the self-service RD Petroleum station. The strange happenings have been going on for months, leaving locals more baffled by the day.

The identity of the mysterious milk enthusiast remains unknown, as does their motive.

Sometimes they leave behind a sole bottle, while other instances have seen as many as six bottles appear at once. The person seems to have been non-partisan in their choice of flavours, leaving behind evidence of their indulgence in chocolate, banana, strawberry and coffee.

The contents of the bottles include brands such as Meadowfresh, Barista Bros, Puhoi Valley and Mammoth Supply - all owned by different companies - and range from half-empty to full.

Poppy Prendergast has been tracking the enigmatic appearances for a few months, and said the bottles were now appearing every few days. They were always found at the pump that serviced trucks, she said, so many thought the person in question must be someone who passed through the town regularly.

"This fuel stop is located kind of in the industrial area, it's a dead-end road, so if you're not going there at peak hours it is fairly isolated," she explained.

"There's a few theories going around ... there's a lot of people saying, 'Well, it's definitely got to be a truckie'.

"The frequent fuel stops, you know, this must be a delivery person, they're coming back and forth. Alexandra, it's a very central town here in Otago, we've got a lot of surrounding towns, so lots of people pass through here everyday.

"The cost of the milk, so this ties in to another theory we've got: These are being given as freebies. This person, this culprit, works within the supply chain for cold, refrigerated items, theory, and they're getting them for free."

She also hypothesised that the person in question was a male.

"Look, I hate to say it but I think it's got to be a man. I've never met any woman that drinks that much flavoured milk."

A range of different flavours have been left including chocolate, strawberry, banana and coffee. Photo: Jay Huddlestone

The bottles often appeared early in the morning, Prendergast said, when visibility was low because of regular fog and winter darkness. Morning frosts and recent low temperatures, often around -3 degrees Celsius, meant the quality of the milk was likely being preserved.

She could not recall whether the petrol station in question had a bin available, but said there were other rubbish facilities nearby.

"So that's the thing - does this become criminal, with all this littering? Are we delving into kind of a Midsomer Murders type territory, minus the murders?"

Another local, Jay Huddlestone, said she first noticed the bottles a few weeks ago.

"Sometimes there's up to six sitting there at a time, normally they are at least half full," she said.

"All the dates are in-date, same as you would expect from a shop.

"It's become rather well known around Alexandra now and it's still a mystery as to who is doing it."

Prendergast worried the mysterious depositer was being spurred on by the attention.

"People are keeping track, people are noticing, this is getting discussed ... I am kind of wondering now if the culprit knows we're watching, and they're doing it for entertainment purposes."

Further adding to the mystery, someone was also removing the bottles. It was unclear whether it was the same person who had left the suspicious items there in the first place.

"Some good citizen is going out there and seeing them and taking them away, and disposing of them appropriately because they don't hang out there for too long," Prendergast said.

"Anywhere from a couple of hours to a full day."

RD Petroleum has been approached for comment.

Main image (Poppy Prendergast): The bottles left included a range of different brands and some were full, while others were half empty.