Joseph Parker will need good defence against Andy Ruiz but is rated a firm favourite to win by the international bookmakers.

Joseph Parker is a massive favourite with international betting agencies to beat Andy Ruiz and win the WBO heavyweight boxing title.

Parker, 24, takes on the Mexican pugilist in Auckland on Saturday night in a scheduled 12-round title fight.

The Parker camp have floated an unconvincing theory about Ruiz, 27, being favourite for the fight in terms of age and experience, despite the Kiwi's home advantage.

The bookies think otherwise. The international betting agencies have followed the early lead of the New Zealand TAB who have Parker a $1.30 favourite against Ruiz who is priced at $3.30 in head to head betting.

There are more generous odds overseas as the fight, with world ramifications, starts to take traction.

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In the UK, Sportsbet has Parker at $1.25 and Ruiz $3.75.

Bet365, another UK betting agency, has Parker at $1.25 and Ruiz at $4.

William Hill also has Parker at $1.25 and Ruiz at $4.

Across the Tasman Sportsbet have Parker at $1.28 and Ruiz at $3.45.

Malta-based Unibet has Parker at $1.25 and Ruiz at $3.75.

In terms of Parker's winning method, the New Zealand TAB favours a knockout or TKO with a stoppage priced at $2.10 compared to a points decision at $2.80.

They have Ruiz at $5 to win by a stoppage and $8 to win by points.

Sports betting analytics company Betfeed.net, which looks at odds and trends for sporting events, rates Parker's winning chance at 74.3 per cent. They give Ruiz a 22.1 per cent chance of winning with the draw at 3.6 per cent.

Both fighters are unbeaten with Parker 21-0 and Ruiz 29-0.

Ruiz's promoter Bob Arum said on Wednesday it was "stupid" to pick a favourite, noting the early blows in the bout would decide such things.

"The first guy to hit the other on the beard, you got to give them the advantage," Arum said.