Rena's striking of the Astrolabe Reef may have damaged one of the region's diving hotspots.

New Zealand Underwater Association president Shane Wasik says the 236m container ship appears to be lodged directly above an underwater cavern.

Astrolabe Reef.

'There's a really big cavern down there that's quite a popular dive spot and the cave goes down from about 15m right down to about 35m and you can dive right down through the reef and it's a really big open cavern.

'It's a really nice dive spot, one of the best in the local area to be honest – a lot of scenic divers would head there.

'It's well known nationally so if you mention the Bay of Plenty to divers they are going to be like ‘oh yeah Astrolabe Reef' – it's a well known spot and a lot of people would want to dive there if they came here for a weekend.

The underwater cave at Astrolabe Reef. Photo: Shane Wasik.

'The boat is sitting on top of it and the worry is that a vessel at that size and the speed it hit it, it might have crushed some of the rock or collapsed the cave – we don't really know, but when you think how many thousands of tonnes the ship is going at 15-18 knots who knows what would have happened to the topography under the water.”

The reef's highest point is pictured here, just to the right of the ship.

Rena is lodged alongside what Shane describes as the 'pinnacle” of the reef. This is the spot where in photos of the ship water can be seen breaking against some rocks.

'Where you see the bit that breaks the water there is a little bit of rock that sticks up and it's kind of a pinnacle of a mountain.

'On its northern side there is a really big steep drop off and it drops off to maybe about 60m and it comes around to an almost tear drop shape.

'Where you can see the water break that is quite a shallow part and it drops down to about 10-15m.”

This forms a kind of ledge near the top of the ‘mountain' and Shane says the pinnacle's south side drops away steadily.

There is a second peak to the reef.

'If you are looking at the peak that's above the water, once it drops down from there then there's kind of a valley underwater kind of thing – a ‘U' kind of shape.”

Shane thinks the ship has passed into and become stuck within the ‘U' of the reef and its lodging there could cause further damage.

'There is the impact damage and depending on how long it takes to get it off there is going to be ongoing movement on the ship as it moves back and forward.

'We are due to get some bad weather next week, so some swell and so obviously that is going to have a toll moving the ship around, but potentially the rock is quite hard there and so the rock is going to do more damage to the ship than the ship is to the rock.”