Detective Inspector Joel Syme said police opted to call in the additional officers after they became aware of the planned event involving gang members and associates.
North Island-based members of the gang crossed the Cook Strait on a ferry and rode south this morning.
"Additional police staff will be deployed in the city over the weekend to closely monitor the behaviour of those attending the event," Syme said.
Syme said police responded to a crash on Friday morning involving two motorcycles travelling to the event.
No one was injured in the crash on SH 1 (Omihi Rd), north of Amberley, which was reported to police at 11.25am.
Stuff has reported the gang is holding a national meeting at its South Island headquarters on State Highway 1, near Burnham, this weekend.
"Our expectations around these types of events are very clear and I can assure the community that we will have resources in place to respond to reports of unlawful behaviour," Syme said.
"We also encourage the public to report any instances of unlawful activity and unsafe driving to us, so we can take appropriate action."
Members of the public are urged to phone 111 to report an incident taking place, or call 105 or visit https://www.police.govt.nz/use-105 to report non-urgent matters to police.
Police officers spoken to by the NZ Herald say their numbers are low, especially when compared to others in the city like the Tribesmen MC, or the Comancheros who recently patched over the rival Rebels and took over their Woolston clubhouse.
In June, the South Island boss of the Mongols Jason Ross was sentenced to 12 years in jail as a result of Operation Silk.
The jury in last year’s trial took five days to find eight members of the gang, including national president Jim Thacker, Mongols national vice-president Hone “H1″ Ronaki, Leon “Wolf” Huritu, Kelly “Rhino” Petrowski, Matthew Ramsden, dubbed the meth “washer” and processor, 24-year-old patched member Kane Ronaki, and Silver guilty on a host of charges.
Thacker was last month jailed for more than two decades.
-Additional reporting Sam Sherwood, NZ Herald