Two Rotorua people remain in serious condition with head injuries after about 200 people attended an open invitation to a series of parties advertised on Facebook and which later erupted into violence.

Police were called to a flat complex in Pretoria Street, where the parties were being held, and King Street, where 150-200 people aged in their late teens and early 20s had gathered late on Friday night.

Six people were taken to hospital with injuries, including two people with skull fractures, one person with a serious face cut, which needed stitches, and one person with a head injury, as a result of the violence, Rotorua police said.

The seriously injured pair remain in Waikato Hospital.

Police are interested in hearing from anyone else who was injured, or witnessed any assaults, or has information about anyone who was armed with weapons.

They are asked to contact Rotorua police on 07 349 9400, or provide information anonymously via Crimestoppers on 0800 555111.

Police issued advice today to homeowners on hosting a party, particularly one that is likely to attract large numbers, to let the police know first so staff can provide safety advice and effective crime prevention if trouble starts.

Police also caution advertising parties on social media and ensure adult supervision at the venue.

Social media and bulk text messaging are two common reasons for large numbers of unwanted guests arriving at parties and causing problems, police said.

Ensure that there is adult supervision and a plan in place in the event of gatecrashers or disorder.

Breaching liquor bans by holding a party in a liquor banned area can lead to fines of up to $20,000.

Anyone under 18 years of age found drinking in a public place can also be issued with a liquor infringement notice and an instant $200 fine.

Adults breaching a ban can also face prosecution.

- © Fairfax NZ News