Rugby League World Cup 2013: Sonny Bill Williams in NZ squad

Sonny Bill Williams

Sonny Bill Williams has been added to New Zealand's World Cup squad after belatedly making himself available.

The 28-year-old dual-code star was not included in the 24-man party named on Monday because of "other commitments".

However, coach Stephen Kearney confirmed second row Williams has now replaced 21-year-old Tohu Harris.

"I'm sorry for taking someone else's spot," Williams told BBC World. "Now that I'm there, don't expect anything other than a full-hearted performance."

Williams started his career in league, but switched codes in 2008 and therefore missed out on a place in New Zealand's squad for that year's Rugby League World Cup, which his country went on to win.

Despite helping the All Blacks win the 2011 Rugby Union World Cup, he said he could not turn down a chance to play in another major competition for his country.

"When I spoke to Stephen Kearney my body was sore and tired," said Williams, who helped the Sydney Roosters win the NRL Grand Final on Sunday, the day before the squad was announced.

"I've not had a rest since 2008 and thought I could go on a vacation with my family but I made a rash decision without thinking about it.

"Afterwards I thought what I would miss out on and remembered how bad I felt in 2008.

"I couldn't look at myself in the mirror so I had to make the phone call and say I was available."

New Zealand are aiming to win the Rugby League World Cup for the second time and Williams has said sorry to Melbourne Storm's Harris, who will now miss out.

Harris helped Storm beat Leeds Rhinos 18-14 to win the World Club Challenge in February and made his international debut two months later, in a 32-12 defeat against Australia.

He has also rejected the chance to be a non-playing member of New Zealand's touring squad for the World Cup.

"I've offered my apologies to him and obviously he is hurting," added Williams, who has played seven rugby league internationals for New Zealand, but is yet to be on the winning team.

"It's just a really tough situation and all I can do now is play as hard as I can and help the boys retain the World Cup."

The competition will be staged in England, Wales, Republic of Ireland and France and Williams could play for New Zealand against the Cook Islands in a warm-up match at Doncaster's Keepmoat Stadium on Sunday, 20 October.

New Zealand then face group games against Samoa on 27 October, France on 1 November and Papua New Guinea on 8 November.