A stairway to heaven? Two seagulls have been killed after being impaled on the end of David McCracken's Diminish and Ascend sculpture at the Christchurch Botanic Gardens.

A sculpture that has killed at least two birds in Christchurch's Botanic Gardens will be modified to prevent any further avian deaths.

Two seagulls have been impaled on David McCracken's Diminish and Ascend staircase since it was installed in a lake at the gardens in 2016.

McCracken said he had been told two seagulls had flown into the end of the sculpture and impaled themselves on the spike.

"It's not really the image you want – dead birds bleeding down the end of the sculpture."

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The Auckland-based artist created the sculpture in 2014. Before being installed in Christchurch, it was on show in Sydney and on Waiheke Island, where no birds met their demise on its spike.

"If somebody has a photo of it I'd really like to see it.

"I can only assume the birds flew straight at it and were not able to see it. Obviously I have to do something about it."

STACY SQUIRES/STUFF The sculpture was installed in 2016.

McCracken said the most likely solution was to cut the tip off and replace it with silicone and paint it to match the artwork.

The sculpture was placed in a lake at the gardens as part of the Scape public art season in 2016.

It was supposed to be temporary, but the Christchurch City Council agreed on Thursday to make the sculpture a permanent feature.

The sculpture cost $192,000 and funding had come from the council's now defunct public art fund, and a grant from the Friends of Christchurch Botanic Gardens.

Maintaining the sculpture cost between $700 and $760 a month.

A council report said the maintenance costs were high because of the sculpture's location in the middle of a lake and the need to regularly clean off bird droppings.

Contractors wade out and waterblast it twice a week.

The maintenance was paid for out of the council artworks budget, but came at the expense of maintaining other art works, council citizens and community principal advisor Brent Smith said.

Some councillors were concerned at the cost to maintain the artwork.

Cr Tim Scandrett said the sculpture was a gift to the council, but gifts could become a "poisoned chalice" because of the costs to maintain them.

"People think this council is an endless bottomless pit of money."

Cr Pauline Cotter said Christchurch was becoming an artistic city and she supported the meaning behind the sculpture.

"We have to diminish in order to ascend, even if do get pooped on at the end."