A Rotorua mother is swearing off Nestle milk formula after getting halfway through a tin before finding a mouse-like object.

Michelle Swinburne was preparing a bottle of S26 Gold Progress 2 for her nine-and-a-half-month-old boy last Sunday afternoon when she found it.

"I put two scoops in, and as I was putting in the third scoop I saw this thing in it, and I thought, 'What the hell is that?'"

She called Nestle, but was told it was after-hours and she should call back on Monday. So she did, only to get through to an answering service who told her someone would call her back. Nestle eventually did, 24 hours later, only to ask her to send the tin back for testing.

"Not once did they ask if my baby was okay, if he was unwell, nothing," says Ms Swinburne.

She posted a photo of the object on Nestle's Facebook page on Wednesday, and the company swung into action.

"They actually rang me twice: 'Is there anything we can do, anything we can offer, ra ra ra, we're very sorry.' It was still a good 10 or 15 minutes into the conversation before she bothered to ask how my baby was doing."

She's taking her boy to the doctor this afternoon to get him checked, saying he's had a bout of diarrhoea.

"My first thought was that 1080 thing -- what does it even look like? I couldn't believe it. Their lack of care, their lack of everything -- it was just like a normal product you complain about, like you get chocolate and it's pre-melted. That was the way they just didn't care."

She's not sure what it is, saying it's "very little" and could be a mouse shrunken down somewhere in the processing chain.

"It doesn't matter what it is -- why the hell is it in my baby's formula? With infant formula, there's no room for error."

Nestle says it's unlikely to be a mouse, because all its formula "passes through a sieve multiple times in production, including as it passes directly into the tin".

"It's very rare for foreign objects to be found in infant formula, as we manufacture our infant formula within a closed system with multiple steps and processes to maintain the integrity of the product," the company told Newshub.

"There are also multiple tests conducted on every batch of formula and detailed records are kept. We take consumer complaints extremely seriously."

According to various online retailers, S26 Gold Progress 2 is made in Singapore using ingredients sourced from New Zealand and the United States.

Nestle acknowledged it was a mistake not to ask Ms Swinburne if her baby was okay.

"Our practice is also to ask about the health of the baby, and to recommend a visit to a healthcare professional if the carer is concerned about the baby. On this occasion, we did not initially ask about her and her baby as we usually would -- this is not how it is meant to be handled, and we have acknowledged this with Ms Swinburne and have apologised to her."

Nestle has sent the object to an independent laboratory for testing, and will check records relating to the rest of the batch to ensure there aren't more foreign objects waiting to be found.

Regardless, Ms Swinburne -- who also has a two-year-old -- plans to switch brands.

"They have sent me a tin in the mail -- it's still brand new and unopened."