Sea lettuce has invaded a Western Bay beach in the middle of winter.

Regular users of the beach at Pahoia Domain say the sea lettuce is unlike anything they have seen before at this time of year.

Then there was the swan poo.

Alanna Ratna said she visited the beach with her children most weekends.

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Last weekend the combination of sea lettuce and patches of swan poo the size of cow pats stopped her kids from playing at the beach.

The sea lettuce formed big "islands" on the beach, she said, and the poo was "disgusting and unhygienic".

A big flock of swans were in the harbour when they visited.

"It's usually such a lovely place for kids to play - quiet and safe and calm. This is dreadful."

There were no swans when the Bay of Plenty Times visited Pahoia Domain on Monday.

However, the metre or so strip of sand not taken by the high tide was blanketed in a thick layer of the bright green ribbons.

Hundreds of islands of sea lettuce floated a short distance offshore.

Graham Walker has lived at Pahoia for five years and walks the beach every day with his dog.

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He was not too worried about the swans, saying they came and went, but said the sea lettuce was as bad as he had ever seen it.

"It's not unusual in summer, but I have not seen it in this volume in winter before."

It was a nuisance, and he would not mind if it was cleaned up - he knew of two people who had slipped on it and fallen.

A regional council spokesman said staff would go and check out the accumulation at Pahoia.

If it posed a risk and was causing a nuisance then they would clean it up.

Regional council environmental scientist Rebecca Lawton said it was "uncommon but not unheard of" for sea lettuce to accumulate this time of year.

"It is likely that the accumulation at Pahoia has drifted into the area by tides and winds from sub-tidal populations of the sea lettuce in other areas of the harbour."

Pahoia was a hotspot for sea lettuce accumulation in summer, she said.

She said the council had monitored sea lettuce levels every two months in Tauranga Harbour since 1991.

The council had supported studies into the factors that could trigger sea lettuce blooms and how to manage them, Ms Lawton said.

Swans and sea lettuce

- November to February: usual season for sea lettuce blooms
- Every two months: Frequency of regional council sea lettuce monitoring
- 1991: when sea lettuce monitoring and swan counting began in the Bay of Plenty
- 4500: average size of Tauranga Harbour swan flock over last decade
- 6200: swan count in January 2017
- 2500: swan count in April 2017
- 90%: typical drop in swan numbers between January and August

Sources: Bay of Plenty Regional Council and Eastern Fish and Game