Vessels without authorisation to be in the exclusion zone around the grounded cargo ship near Tauranga should also avoid the area affected by the oil slick.
Regional Harbour Master Carl Magazinovic reminded people that a 1 kilometre (0.62 mile) exclusion zone around Astrolabe Reef remained in force.
"We have had confirmation from Maritime New Zealand that there is an oil slick stretching about 2000 metres from the cargo ship Rena, and vessels should keep well clear," Mr Magazinovic said.
"Vessels that approach the site and the slick will potentially disrupt the response effort, and are putting themselves and others at a safety risk.
"This is a serious situation, and people who are not directly involved in the response effort should not be in the area."
Mr Magazinovic reiterated that significant penalties applied for breaching the exclusion zone.
"We are continuing to monitor the area, and will have no hesitation in taking action against anyone who fails to adhere to instruction.
"I remind everyone that anyone found breaching the exclusion zone could face a fine of up to $20,000.
"We expect the exclusion zone to be in place for some time," he said.
The fully laden container ship, the 236m Rena, grounded on Astrolabe Reef, approximately 11.5 nautical miles from the Tauranga coast, at 2.20am yesterday.