Ports of Auckland workers warned against contracting jobs lest they will be compelled to leave the country, the Maritime Union says.

The port reportedly plans to make up to 320 union workers redundant and replace them with contractors. The union has issued a sixth strike notice with these developments.

Port chief executive Tony Gibson says the move will improve labour productivity, but Maritime Union national president Garry Parsloe says the workers will not accept new conditions.

"If the Ports of Auckland continues to attack its own workforce, they'll go somewhere where they can get a better deal - like Australia."

Parsloe says the huge investment that has been made in training a skilled workforce will be wasted if workers leave the port.

In reaction to the strike notice, Gibson said it was "highly irresponsible" of the workers to take such an action.

Gibson said the sixth strike would take place for a 24 hour period from 7am on January 31 to 7am on February 1.

"This sixth strike will do nothing to end the dispute other than to strengthen our resolve to sort this out once and for all," Gibson said, adding the port would try to stay operational during the strike by using non-union staff.

"We will also continue to work with customers, other ports and the road transport industry to help minimise the impact of any stoppages," Gibson added.

Meanwhile, Council of Trade Unions (CTU), represented by Wellington-based CTU president Helen Kelly, says the port dispute has become its top priority.

Kelly is in Auckland this week to offer support to the Maritime Union of New Zealand after a six-hour mediation session with port management last week failed to resolve the dispute.

Kelly, who attended the mediation, said port management acted in an ''unprincipled way,'' as it reportedly refused to build a relationship with the union, thereby crushing the possibilities of negotiation.

Meanwhile, the International Transport Workers Federation - a global federation of 751 transport unions representing over 4.6 million workers - was very aware of the dispute and had offered their support, TVNZ reported.