Greenshields (27), an administrative assistant at Moana Pool, broke two records on the first night of swimming at Moana Pool in the women's aged 25 to 29 grade.
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She won the 50m breaststroke gold medal in a record 34.42sec and followed it up with the 200m individual medley in 2min 23.96sec.
Greenshields competed for Great Britain in the 200m breaststroke at the European championships in 2005 and 2007 and then took a break from swimming for five years.
She broke Scottish senior records in the breaststroke events and a Great Britain junior record.
Greenshields returned to swimming when she settled in Dunedin last year and is a member of Gennadiy Labara's Osca squad.
She will be married to her Dunedin fiance in her home town of Peebles before returning to Dunedin intending to break more swimming records.
Kurt Crosland (27), another member of the Osca squad, was also in record-breaking form and set New Zealand masters marks in the 100m backstroke (55.32sec) and 50m breaststroke (30.87sec).
Veteran Lenore Sonntag (74) smashed the 200m individual medley record by 45sec in the women's 75 to 79 grade with a time of 4min 19.58sec. The old record was set by Joan Bidwell (North Shore) two years ago.
Sonntag also lowered the 50m butterfly record to 57.01sec.
Southlander Alistair Mackay (84), the oldest swimmer at the Masters Games, set a record in the men's 85 to 89 50m breaststroke with his time of 1min 08.29sec.
Lynnette Hines set a record in the women's aged 65 to 69 50m butterfly (45.17sec).
There were also two men's national masters records in the relays. They are classified by the combined ages of the four-man team.
In the 160 to 199 grade, the team of Crosland, Kieran Garbutt, Steven Sexton and Peter Graham narrowly beat the record with a time of 2min 01.22sec.
In the 240 to 279 grade, the team of Stephen Clark, Steve Prescott, Mark Johnston and Forbes Sonntag swam 2min 24.87sec to beat the old record by 11sec.
In the women's 70 to 74 grade Anne Cresswell, from Benalla in northern Victoria, won gold medals in the 50m breaststroke (58.97sec) and 50m butterfly (1min 02.69sec).
She took up indoor rowing to get fit for swimming and has become an Australian masters representative in that sport as well and has broken eight world records.
She won indoor rowing gold medals in the 500m, 1km and 2km in Dunedin this week.
Margaret Wilding (England), who won a bronze medal in the 100m breaststroke for Great Britain at the 1956 Olympic Games at Melbourne, won her specialist event in 1min 36.43sec.
