A New Zealand man seeking to join his partner in the United Kingdom has never met their 5-month-old son following visa and passport issues.

Luke, 29, has been separated from his British partner Simone Brookes and their three children for nearly a year owing to a series of delays and alleged errors by UK immigration officials.

The couple had lived together in Paeroa for 10 years, where they had their two daughters Bella, 7, and Raia, 4.

They planned to move to Britain before their third child, Raiden, was born in May.

"It is incredibly frustrating, depressing," Luke told the Herald.

"We have just been waiting and waiting, I miss my girls, and I still haven't met my boy."

The couple came to the United Kingdom initially on December 17 last year, but Luke was deported nine days later on Boxing Day as his visa was not in place.

They filed the appropriate forms, while Brookes stayed in Britain with their children as she was pregnant at the time, and they thought it would only take a few months for the visa to be sorted.

Over the next few months they had a series of paperwork issues, and eventually applied for a holiday visa out of desperation for Luke to be at Raiden's birth, who had heart complications.

Brookes said this was denied as officials thought her partner would not return to New Zealand.

After consulting a solicitor, Luke applied again for a visa in April, paying more than $1000 to have it fast-tracked due to their son's heart condition.

The couple allege the Home Office then made a series of errors and delays including losing Luke's passport and even claiming to have never received the application.

When the Home Office eventually sent Luke his passport and visa at the end of September, he discovered the entry stamp to the UK had already expired.

Brookes said the process had driven them both into states of depression.

"We've both faced depression as a result of it and just need our family back together.

"We miss him massively and our son still hasn't met his dad in real life.

"I have tried to be tough for the family but it has been the hardest thing I've ever done.

"My kids have been given so many broken promises thinking Luke is coming back, and have been let down so many times when he's still not here."

Their solicitor, Danielle Blake, told the Independent it was one of the worst cases she had worked on.

"It's extremely frustrating. They took a month to even realise they had the paperwork to begin with.

"The Home Office determined back in July that they were going to approve the application – we had confirmation of that then – but it's taking them so long to actually get the visa issued.

The Herald has made a request to the Home Office for comment.