Two people in a life raft have been rescued by crew on a brand new Coastguard vessel south-east of Mayor Island this afternoon.

A brother and sister on a two-day trip from Tauranga out to Mayor Island sent out a mayday call at 1.30pm this afternoon after their nine metre launch Mistress started to sink.

The pair were safely rescued by crew members on Auckland Coastguard vessel Trillian Trust Rescue.

The rescue boat was on its maiden voyage from Tauranga to Auckland.

Coastguard New Zealand spokesperson Julia James says both individuals are uninjured but shaken.

She says conditions were good out on the water but the vessel just started to sink from underneath them.

"They quickly jumped into their 7-foot inflatable, and they did a really smart thing by grabbing their EPIRB on the way."

The call from the distress beacon was picked up by the New Zealand Rescue Coordination Centre at 1.30pm.

The TECT Rescue Helicopter arrived at the location of the beacon signal and began a search of the area, says a statement from the TECT Rescue Helicopter.

A life raft firing a flare was sighted by helicopter crewman around 2.09pm and the helicopter was joined on the scene by an NZDF King Air.

The two aircraft stayed on scene providing air support and tracking the life raft until the Coastguard arrived.

Auckland Coastguard vessel Trillian Trust Rescue saved two people on its maiden voyage. Supplied image.

"While this was all happening our Auckland Coastguard Unit happened to be taking back a new vessel they have literally just picked up from the boatyard in Tauranga.

"They heard the mayday call when they were near Mount Maunganui and were on their way back to Auckland.

"It is quite amazing – the crew just happened to be in the right place at the right time," says Julia

She says the pair are physically okay but "pretty shaken" about what happened.

The reason for the sinking still hasn't been determined.

"The boat was in good condition and the weather conditions were good – they don't know what happened for it to sink," Julia says.