The company behind the South Island's biggest port is being accused of keeping workers "out of the loop" on its $800 million upgrade plans by one of its unions.

This is despite an assertion by its chief executive that it briefed its entire workforce several months ago on a proposal, described as "the most ambitious port project in a generation".

Lyttelton Port Company (LPC) will spend $821m expanding its container terminal and building a new 388-metre deepwater wharf at Te Awaparahi Bay.

The expansion plans include a five-hectare container terminal, four new ship-to-shore cranes and semi-automated gantry yard cranes, with the work expected due to be completed by 2031.

The company said in an announcement on Monday the project would help meet growing demand from South Island exporters and accommodate bigger ships.

LPC chief executive Graeme Sumner said 92 percent ships being built today could not fit into Lyttelton Harbour.

"The reality is that if we didn't do this, those larger container ships would be unable to arrive here," he said.

"We are running out of land as well in order to be able to handle the number of containers that are coming through."

The announcement also follows a recent decision by Christchurch City Council's investment arm and LPC owners Christchurch City Holdings' Limited (CCHL) to reject an unsolicited proposal by global port operator DP World and three Canterbury rūnanga to lease the port's operations.

Sumner told reporters at Monday's announcement it would need to employ an additional 50-100 people during development, although long-term, automation would ensure greater output at the port with similar staffing levels seen at present.

When asked if the unions had been briefed on the plans beforehand, Sumner said the company made a presentation to all staff about eight or nine months ago.

"The joke I suppose amongst the staff is maybe you could talk about it a little bit less. It is something that we see as being strategically absolutely vital," he said.

"So we've taken our staff along with us the whole way."

The Rail and Maritime Transport Union's (RMTU) Lyttelton branch secretary Mark Wilson rebuffed this suggestion, arguing they had called for better engagement over the project to no avail.

"We certainly haven't been brought along the whole way," he said.

"We have concerns around the expansion of the port will mean for our members. There's certainly a component of automation that could be implemented in his new wharf."

The union would have liked to be more involved with the business case to understand the reasons for needing to do the upgrade, he said.

A lack of transparency had extended to the community and the mana whenua who had also "really been left out of the loop," he said.

"We need a management structure that actually treats the workers, mana whenua and the community as strategic partners in the operation of the port.

"We have no relationship at the present time and it's quite unfortunate and quite disconcerting."

The project would be funded mostly through additional borrowing alongside an equity contribution from CCHL.

LPC recorded a net debt of about $202m at the close of the 2025 financial year, which would mean higher costs for port users.

"We will be talking about that later in the year and releasing a price pathway going forward so people can see how it's going to be funded," Sumner said.

"They will be reasonable. But at the end of the day, it's a substantial lick of infrastructure that needs to be funded and it will have to be supported."

The company reported a record net profit after tax of $35m for the last financial year, marking a 40 percent increase from $25m the previous year.