Sonny Bill Williams breaks boxing's biggest rule as he makes stunning confession about his fight with Paul Gallen - who wants to punch on in his UNDERPANTS

Sonny Bill Williams has made a stunning confession about his long-running feud with Paul Gallen ahead of the pair's highly anticipated heavyweight showdown: he's been afraid to fight his long-time rival.

After years of back and forth and stalled negotiations, the former NRL stars will finally get to settle their differences at Sydney's Qudos Bank Arena on Wednesday night.

Gallen has long accused Williams of running from the fight and waiting until the Sharks legend is long past his athletic prime.

When asked at Monday's press conference who was avoiding the bout, Williams admitted it was him. 

'Me. I've been running,' he said.

'But we're here now.

'I'm just laser-focused on Wednesday night. I couldn't care less about today, about tomorrow, and what's going to be said. I'm laser-focused and I know that Wednesday night, if I'm laser-focused on what I need to do, I'll show how limited this guy is.'

Williams then clarified his position, saying fighting Gallen just hasn't been on his mind.

'I have not been running at all. I just haven't been really, to be honest, thinking about Paul,' he said.

'He's been fighting, fighting. He was fighting for many years when I didn't step foot in the boxing ring. I was playing rugby around the world. I didn't know if I was going to get back into the boxing ring.

'The one thing I've always said is, 'Yeah, I might not be the best boxer, but I know I can beat this guy'. I've always said that, and we're here now.'

Williams, 39, goes into the fight as the slight favourite despite being knocked out in his last bout and having less experience than the 43-year-old former Blues skipper.

Gallen blasted Williams for previous comments he'd made about his character.

'People in glass houses shouldn't throw stones,' said Gallen.

'This bloke has said he really dislikes me because of my cushy job - we've got the same job, we are employed by the same people, you idiot.

'Then he wants to talk about drugs. This bloke is an admitted drug user, you admitted to using drugs to cope.

'Then you want to make up a lie about a $10 barbecue... don't you do food reviews Sonny? Do you charge people for your food reviews?

'Me asking blokes to tip in $10 - which isn't true by the way - is frowned upon. You are worth millions of dollars, do you charge people for food reviews? Do you charge people to do food reviews? Yes or no?

'You go to small business, you're supposed to help people in the community, you go to small businesses but you charge them between $2000 and $3000, I've been told.

'I've got a recording in my phone right now from someone who told me you charge... you are a scumbag, you are a disgusting human being.

'You walk around like you help the community and do this and that for people when you go and charge small business thousands to 'help them' - you're a piece of s**t.'

Gallen also hit Williams for six when he suggested the pair fight in a locked room wearing hardly any clothing.

The former All Black told the crowd at the presser that 'it'd be nice to remove all these chairs and get it [the fight] done right now'.

'I'd love to be locked in a room and see who walks out, no rules, in our undies, see who walks out,' Gallen replied.

A shocked Williams then said, 'That's a bit too far, man. A bit too far.' 

Aussie boxing legend Danny Green said he's looking forward to Wednesday night.

'It's going to be intense when the bell goes. These boys both really want to win. Sitting next to these blokes, I'm even more excited,' he said.

'I can see how attuned they are to what they want to do – and they want to knock each other out. I can't wait to sit ringside and watch these two belt each other.'

The pair will meet again on Tuesday for the weigh-ins.