Fans are being urged to prepare for heavy rainfall at Wednesday night's State of Origin rugby league game in Sydney.

A crowd of 70,000 is expected to converge on the ANZ stadium at Olympic Park for the first in the three-match series, with fans being urged to take public transport to avoid wet weather traffic snarls.

Spectators should take advantage of the express train and bus services included with every ticket to avoid heavy traffic, ANZ Stadium CEO Daryl Kerry said in a statement on Tuesday.

The match between NSW and Queensland is due to kick off shortly after 8.10pm (AEST), with police warning they will crack down hard on any drunken or anti-social behaviour at the match.

Last year, 40 people were ejected from the stadium and six were banned from returning to the stadium for 12 months.

"The majority of Origin fans enjoy these matches without incident," police Assistant Commissioner Peter McErlain said in a statement on Tuesday.

"Nonetheless, I make no apologies when coming down hard on a select few who want to break the law."

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