NASTY PROBLEM: Pacific Wallcoverings general manager Andreas Paxie on the roof where seagulls are making a mess.

Huge flocks of seagulls are dropping animal parts, bones and condoms on industrial roofs in Porirua, leaving businesses considering whether they should move on.

Up to 20,000 black-backed gulls can swarm over a street at one time, dropping rubbish and causing roofs to corrode under inches of urine and excrement, business owners say.

Pacific Wallcoverings on Mohuia Cres in Elsdon has had to send staff home because of the stench and health risks of a yard covered in rubbish and filth.

General manager Andreas Paxie said the problem was so bad he was tempted to move out of the city.

"It is seriously causing us to review whether Porirua is really a viable location for our business after 50 years," he told his landlord in an email.

Porirua City Council had known about the problem for years, but had not acted, Paxie said.

"If anything, all the work that the council does in promoting Porirua as a business destination is largely undone by the complete lack of action regarding seagulls.

"These gulls are flying rats. If 1000 rats headed out of the Porirua landfill each night and made their way down the street to the Porirua council building, bringing treasures from the tip with them, the council would be giving this issue a higher priority."

Bird expert Elizabeth Bell attended a July 12 meeting between business owners and council representatives.

A report she prepared said removing birds from roofs or scaring them away were only temporary measures.

The best approach was to conduct a five-year leg banding study, costing $189,000, to identify where the problem birds nested, and remove them at source.

Good tip maintenance and shooting birds at the landfill, which was done at Spicer, also helped, Bell said.

Pacific Wallcoverings building owner Allen Shanks said the gull problem was now seven times worse since businesses approached the council for help in 2006.

"They missed the boat years ago, when they could have tackled a much smaller problem."

With such high rates already, property owners could not be expected to foot the full bill for a study and cull, Shanks said.

"As property owners, we see it as a city-wide issue."

Wellington Windows and Doors director Alex Laplanche has had to remove staff from the top storey of his Mohuia Cres building because poo on his roof was causing a smell and maggots were dropping down from the ceiling.

"It was putrid."

Even if a banding study delayed action for a few years, Laplanche urged the council to move ahead with it.

"Anything. Something needs to start now. Why wait?"

Money was partly the reason for inaction in the past, and identifying responsibility for the gull problem was tricky, said Peter Bailey, the council's general manager of asset management and operations.

It was not just the tip that attracted gulls - people dropping food was a big problem.

That was proved by the lack of seagulls in the CBD after Christmas, when nobody was around, Bailey said.

The council and Porirua Chamber of Commerce were investigating the problem, but at present, action consisted of upping the daily cull at the tip and scaring methods, chamber chief executive Holly Thompson said.

Mayor Nick Leggett said the issue had never come before council.