Kingdom: Plantae
(Unranked): Angiosperms
(Unranked): Eudicots
(Unranked): Rosids
Order: Rosales
Family: Rosaceae
Subfamily: Rosoideae
Tribe: Sanguisorbeae
Subtribe: Sanguisorbinae
Genus: Acaena
Species: A. anserinifolia
Binomial name: Acaena anserinifolia
Synonyms: Acaena pusilla var. pusilla, A. pusilla var. suprasericascens A. viridior. 
Common names: Bidibid, Cockle button, Hutiwai, Piripiri, pirikahu, piriwhetau, huruhuru-o-hine-nui-te-pō, kaiā, kaikaiārure, kaiārurerure, kaikaiā.

Acaena anserinifolia is found from lowland to lower subalpine forest margins and in shrublands throughout the North, South, Stewart and Chatham Islands. It is naturalised on the Auckland and Campbell Islands. It flowers from October to January and fruits from December to April.
It was used medicinally by the South Island Māori. The leaves were used to make an infusion which was taken as a diuretic (O'Carroll 1884). This infusion was applied to open wounds and rubbed on contusions.

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Photographed North Egmont late January.
Acaena anserinifolia Mt Egmont-001.JPG

Photographed January Arawata Road, South Taranaki, next to Mt Egmont/Taranaki Reserve.
1-Acaena anserinifolia Piripiri -001.JPG

1-Acaena anserinifolia Piripiri .JPG

Photographed at Otari-Wilton's Bush reserve, Wellington
Acaena anserinifolia-002.JPG 

Acaena anserinifolia Piripiri -001.JPG

Acaena anserinifolia-004.JPG

The fruit with the visible spear like hooks.
Acaena anserinifolia-005.JPG

Thanks to Wikipedia for text and information