English reggae pop group UB40's Nelson concert has been made R18 just hours before it was set to begin at Rutherford Park.

The group's NZ tour has been fraught with complications caused by bad weather, with Tauranga and Mount Maunganui shows both being cancelled.

The Nelson concert was set to begin at 4.30pm today but attendees plans were upended by the surprise announcement made at 2pm.

"Unfortunately, there has been an oversight with our liquor license application that has only been identified today, we regret to inform you that the event will now be R18," the post read.

Neptune Entertainment promised a full automatic refund including ticket booking fees for those with child tickets and said adult tickets could be refunded but only if they were purchased alongside child tickets.

The refund window will be open until midnight Sunday.

Commenters on the Facebook post expressed frustration and disappointment, with many feeling they should have banned alcohol instead so fewer familys' plans were disrupted.