Hackers have hit the New Zealand TAB in a cyber attack at one of the busiest periods of the betting year.

Users of the bookies' website and mobile sites have reported issues when trying to place bets since the attack began on Boxing Day.

The sites have experienced intermittent issues with screens freezing or running slowly, forcing punters to the phones to place bets and then overloading that network with high call volumes.

TAB says staff have been working around the clock to tackle the issue and offered their apologies to those affected.