Relationship counsellors say they are bracing themselves for a "post-lockdown reckoning" similar to that usually seen after Christmas.
Steph James, a counsellor in rural Wales, said couples "thrown together in a stressful situation" could trigger a "surge" in relationship breakdowns.
A UK-wide poll of 2,000 people at the start of lockdown found 23% said it was placing pressure on their relationship.
Relate Cymru urged people to seek help and not let problems "fester".
The Relate survey in April found 29% of those questioned in Wales said lockdown was exerting extra pressure on their relationship.
Around 20% of respondents in Wales said they were having doubts about their future together, compared to a figure of 12% for the UK as a whole.
Of those questioned across the UK, 42% said they were finding their partner irritating and 36% argued more.
Ms James said: "We may see a surge as we do after Christmas. Couples are thrown together in a stressful situation like this.
"Many can't leave, and there may come a lockdown reckoning where all the issues come to the fore.
"They are trying to get through, stiff upper lip. But as we get to some kind of normal, relationships will break down."
However, confidentiality issues have meant no counselling for those in abusive relationships, despite a "huge increase" during lockdown.
Ms James described some couples opening up more on video conference sessions from the comfort of their living room, and one couple becoming closer despite the husband being in Dubai and wife in Wales.
'It was highly stressful'
Lauren (not her real name) returned to her native Cardiff for a break before lockdown, she and her boyfriend of four years having just been furloughed from their jobs in England.
But when the lockdown kicked in and the pandemic stretched on, the pair found themselves reflecting on the relationship.
They eventually decided on an amicable split.
The 24-year-old said: "It just made me realise that it would be possible to have a different life without being in a relationship, and I don't think that I would have realised that had it not been for lockdown.
"I think it's definitely a step in the right direction for both of us, so it's sort of a weird blessing in disguise."
Lauren said the pandemic had certainly added strain to the relationship.
"It was highly stressful finding out that we weren't going to be getting an income and trying to do a joint Universal Credit application," she said.
"And so trying to sort all of that financial stuff out, while still contributing to the same rents and bills and having the trouble of not really being happy in the relationship, that was a massive pressure."
While the break-up has been tinged with sadness, Lauren felt it had ultimately been healthy for both parties.
"I don't think I realised how much my headspace was taken up by thinking about how my decisions might affect someone else," she said.
Ms James, who is based near Cardigan and works across mid and west Wales, said farming couples in particular had struggled, with the demands of rural life taking their toll.
"It's already difficult running a farm and you add into that home schooling and getting the children to knuckle down.
"We've also had the beautiful weather and them wanting to go outside, with a lot of rural areas isolated, the five-mile rule and no access to childcare."
"There has been a huge increase," Ms James said.
"Couples have been thrown together and abuse issues have manifested themselves. Home should be a safe area, but often it isn't."
However, counselling for couples in this situation often involves chatting to them individually to get to the bottom of the abuse issues.
"If one of them is unsafe, [chatting via video conference] with the other partner in the house, the more vulnerable person could be in danger," she added.
"We don't want to put anyone at greater risk, so have to prioritise safety."
Ms James noted that counselling via video apps and telephone had seen many benefits, including people more open to something often seen as "taboo" from the comfort of their homes.
While there was a drop-off in counselling at the start of lockdown, more couples have now been engaging with it.
Lockdown has not been bad for all though, as Ms James points to the couples that have "pulled together", had less distractions and been able to focus on their relationship, as well as realising all their problems were trivial in the face of the pandemic.
There have also been the emergence of "turbo relationships" - those who were in the early stages when lockdown started and then moved in together, which could have been "a recipe for disaster".
"You're together 24-7, not going out with friends, so building very strong foundations," Ms James said.
"Time will tell, these are unprecedented times. Will they flounder on foundations, or stay strong? One core aspect will be spending time out of lockdown."
Relate Cymru's manager Val Tinkler urged people to seek help now and not let problems "fester until things get irreparably bad".
"We always see a peak in people seeking relationship support after Christmas, when spending unusually long together brings issues to the surface," she said.
"Add to that the current extended period of isolation, worries about job security, finances, how to juggle work with childcare and uncertainty about the future - and it's clear why we're expecting a relationship reckoning."
Relate launches its first ever Relationships Week from 20 to 26 July, for those struggling to cope as they come out of lockdown.