A container ship entering the Tauranga Harbour had to alter its course to avoid hitting a small yacht on June 5. Supplied / Chris Cousins

An investigation has been launched by Tauranga Harbourmaster after a container ship entering Tauranga Harbour was forced to alter its course to avoid hitting a small yacht.

Eyewitness Chris Cousins from Te Awamutu said on Sunday about 1.35pm he and his family were walking down the 4WD track on the Matakana Island side of Mauao.

He said as they walked along they suddenly heard a ship loudly sounding its horn in an intermittent "blast" went on for about one to two minutes.

"It was not something I ever heard before and as we rounded the corner I could see a little white yacht sailing along in the channel at the same time as the container ship was entering the harbour.

"Then I saw the ship suddenly take a hard right turn heading towards Matakana Island to avoid hitting the little yacht. The ship seemed to veer off its course at a funny angle.

"I thought to myself, 'oh crikey someone's in all sorts of trouble' but I couldn't see anybody on the deck of the white yacht.

After the "close-call" the yacht carried on sailing away and after the ship went around the yacht it carried on its journey, he said.

"It's extremely lucky, hugely lucky in fact, that the ship was able to change direction in time... I estimate the closest point between the two vessels got before the ship veered off its course was less than 100 metres."

Cousins said after the "near-miss" he saw two other boats come into view - a black boat sailing some distance behind the yacht and another boat that came alongside the black boat for about 30 seconds to a minute.

"I'm quite surprised neither boat followed the little yacht and tried to stop it from leaving.

"I'm sure I was not the only one on Mauao who saw this near-miss and health and safety officials need to get involved and decide what to do next to prevent a similar incident."

Tauranga Coastguard's unit manager Dane Robinson said none of the Coastguard's boats responded to the alleged incident and he was unaware of the matter until approached.

However, he said it was normal practice for Coastguard radio operators to send out a warning alert to other boaties 15 minutes before ship arrivals via VHF channel zero-one.

A Port of Tauranga spokesperson said the authority was aware of the alleged incident and the Harbourmaster was investigating.

The Harbourmaster has been approached for comment.