The regional council is investigating complaints that a boat anchored in Anzac Bay pumped contaminated bilge water among swimmers.

The boat, Lola May from Kaiapoi in Canterbury near the mouth of the Waimakariri River, was found swamped in Shelley Bay on Monday morning after it was caught by the tide.

Families play in the water at Anzac Bay, while the Lola May remains moored in the background. Photo: Elaine Fisher.

It was pumped out and refloated by the Waihi Beach Coastguard and Athenree Volunteer Fire Brigade, and has been anchored in Anzac Bay ever since.

The pollution is believed to have happened on Tuesday or Wednesday, but the regional council pollution hotline wasn't contacted until Thursday, says investigating officer John Morris.

'We believe for some reason they didn't complain at the time that it happened, so we only got this information yesterday,” says John.

'We went out today to have a few words with him, but there's no visible signs in Anzac Bay of the diesel.”

It is thought the former fishing boat's bilge pumps activated automatically, clearing the remainder of the seawater from the former fishing boat's engine room and bilge, after the boat was refloated on Monday.

The Lola May is believed to be making its way to Auckland, but will stop off at Whangamata where the couple on board plan to ditch the contaminated mattresses and squabs doused when the boat swamped.

The launch grounded at low tide on Sunday night and filled with water on the incoming tide.

'If you don't know that part of the harbour you will end up in trouble,” says Waihi Beach Coastguard skipper Tim Watts. 'It goes from about seven metres (depth) to one metre, in about two metres.

'He anchored in seven metres of water and of course the tide came in and swung him round into about one metre and he tipped over on his side.

'He tried to moor in Shelley Bay, but as the tide came in and out, he obviously got up on the sand and filled up full of water.”